Radio Glen Transmitter Information
Technical Data And History Of the
X-Model Prototype Transmitter
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Status: Draft
V2.0 30th November 2002
Change History:
0.0 19 November 1997 Draft Created
1.0 16th January 1998 Approved Released To Glen
1.1 6th August 1998 working Adding a bit at end
2.0 29-NOV-2002 Approved Web Published with some edits to
explain the history.
1. Introduction,
Copyright Notice And Disclaimer
This document describes the transmitter system
supplied to Radio Glen for testing as designed and constructed
by Henry Walmsley.
The transmitter is supplied in
prototype form and has the physical limitations of an instrument
constructed using prototyping techniques. The transmitter has
been designed to conform to the Radio Authority and Wireless
Telegraphy requirements but the user must realise that this
transmitter has not passed formal type approval tests. The
operating station licensee and the technical manager bear the
responsibilty of ensuring that the transmitter is not used in
such a way as to cause interference to other wireless services
or as to cause danger due to electrical shock or fire. Due to
the increased power output capability of the transmitter it is
especially essential that the signal distibution system is
maintained in good condition and checked regularly.
2. History
Over two years from 1991 to 1993 I was
technical manager of Radio Glen based at Glen Eyre Hall at
Southampton University. The original radio distribution system
based on a traditional RF induction system was still in use
which was originally installed around 1978. This used a
low-level (about 2Vpp) AM radio signal sent along standard
coaxial cable with 30V DC. The small transmitter boxes in the
attics and on the roofs of the buildings would use the 30Vdc to
amplify the signal and feed it to a loop aerial of about 1m
diameter. This gave enough magnetic output to penetrate just
down to the lower floors of most of the buildings.
The problem with the system was one of
maintenance and inaccessibilty. Each of the small transmitter
boxes had its own parallel tuning capacitor and output level
control. This meant that if the level was set too high the
transmitter would distort, if it was too low the output would be
poor. If any changes were made to the system, then to ensure
optimum performance, each and every small transmitter would have
to be readjusted for best output. As some of the units are on
top of four storey buildings with no roof access I decided to
change the active transmitters to a passive system.
Based on measurements I made of the
loss in the cable it seemed that if you put a high level signal
into the cable at the station end there would still be plenty
left 1km down the line to drive a passive loop connected across
the coax quite well. The problems of setting levels and of
transmitters breaking down on top of roofs were eliminated. But
one problem was that the power of the transmitter that I built
then was quite low, about 0.6W. This was fine at the station end
of the cable, but output was not so great at the far end.
(Though still generally better than the previous system if the
cable was intact.)
Also, the transmitter modulator used
was the same unit from 1978. Though it inarguably worked, it was
a basic affair which formed part of the original Radio Glen
third year project.
About 1995 the station moved from F
block to the new station near the old terraces. The same system
was pressed into service with the power amplifier of the
transmitter staying in the F block technical cupboard and the
modulator moving to the new studio and sending low level RF
though the conduit back to reception and on to F block.
I thought it was about time the
ye-olde transmitter and PA were replaced with something which
sounded better and had more power to drive the passive loops.
Though the project to build this started out as a replacement PA
only, I got a bit enthusiastic and decided to do a new modulator
as well. The design brief that I set would be that it would be
as cheap as is humanly possible, as idiot proof as possible, and
built entirely from Maplin components for easy replacement. The
Maplin requirement fell by the wayside about 6 months into the
design when it finally became clear that none of the transistors
Maplin stocked were capable of good RF output. In the design as
it stands, 90+% of the components are available from Farnell
components, with only the toroidal iron powder cores being
difficult to find in Britain. (They are easily ordered from the
USA however.) The use of a synthesised frequency generator
completely eliminates the traditionally difficult question of
where to find the crystal.
3. Present And
Future Status
The original modulator has only the
obligatory audio hard limiter and low pass filter prior to the
actual modulator itself. Also this filter was set to the older
standard of 4.5kHz bandwidth limit. The current system may have
an Alice active limiter before the transmitter input, but that
is the limit of the audio processing. The new modulator unit has
three split bands of 2:1 audio compression followed by summation
and a stage of overall active limiting. A hard limiter follows
to cope with transient peaks and the resultant signal is
low-pass filtered prior to the modulator. This should result in
a great increase in perceived quality and loudness, as well as
being vitually bombproofed against the most enthusiastic student
presenter. The modulator and PA are in separate 19inch boxes so
that the current division between studio and remote technical
cupboard can be maintained. The PA has two outputs both capable
of providing 10 Watts AM into the 50ohm system, and both RF
input and output on the PA are isolated from the mains earth to
avoid mains earth contention, and the kilometer wide earth loops
currently endured.
3.1 Into The
21st Century - Friday 29th November 2002
Well, the induction
system is long since gone thanks to Wiggy's sterling efforts
back in 1999 to get the low power free radiating license and a
real transmitter. Wow, a real transmitter and aerial system from
Radica, Luxury! For a while I thought that I would modify these
units so that if the real transmitter failed or was reposessed,
they could hook this up to the proper aerial and run on reduced
power. I went as far as changing the frequency output of the
modulator box and checking that the output from there would pass
broadcast specs, which it would just about with the bit more
audio filtering that I added. The PA was a different matter
though. I'm not an RF engineer by training or at heart and the
PA was simply not linear enough. I wasn't going to start
building a new linear PA or making a high level modulated
version so that project bit the dust. The old PA TX01 was
scrapped, but the modulator still exists and I use it's meagre
2Vpp 50Ohm output sometimes for nostalgic 1287AM broadcasting
around the house. Strictly speaking I suppose Radio Glen still
own it. If their Radica setup breaks and they can't afford to
get it fixed, you could probably use this modulator with a bit
of a linear booster to drive a modified 160m band amateur radio
linear amplifier. In practice, you proably wouldn't bother. I've
put the design up on this site as an example of how to do some
nice simple low cost and highly effective split band audio
compression, and also how to make a nice simple frequency
synthesiser to avoid using crystals. Turn the split band
compressor into a five band version and you would have some
seriously good sounding AM processing. The three band version is
a bit light on the bass and treble. I wonder if I can find some
old recordings? I suspect so... The schematics show the modified
version which has an extra couple of poles of audio filtering
bolted on and will just about pass broadcast specs.
Readers interested in AM audio processing and compensation for transmitter properties should read the technical manuals for the Orban Optimod series of processors whose basic principles are copied for this project in simplified form. SURGE have recently acquired a 9100B processor. Orban Optimod Technical Manuals For The 9100B AM Processor
4. Technical
Specification: Modulator Unit
Power: 240V mains
Audio Input: Stereo standard line
level 0dbm 775mVrms on XLR sockets balanced input.
Indicators: Mains neon, 5V rail LED,
22V rail LED, crystal oven heater LED
Controls: Mains on/off rotary switch
Frequency Ref: Ovened crystal driving
PLL frequency divider
Ouput frequency: Supplied as 1602kHz
+/- 2Hz for University Radio Glen.
(now modified to 1287kHz)
RF output level: Approx 2Vpp with 80%
modulated AM signal
Output connector: Female BNC
Case: Standard 19” rack mount ?U
Mod bandwidth: -3dB at 6kHz from
carrier, -40dB at 9kHz as required by WT regs.
Modulation limiting: Will not exceed
100% modulation with transient +10dBm 1kHz tone burst.
5. Technical
Specification: PA Unit
Power: 240V mains
Input: Variable 0.5 to 2Vpp with 80%
modulated AM signal
Indicators: Mains neon, 30V rail LED,
Unregulated supply LED.
Controls: Dual on/off for each
section. Dual input attenuators for each section.
Ouput: 10W AM into 50 ohms from each
section.
Input connectors: Female BNC
Output connectors: Female BNC
Input impedance: Approx 1kohm each
section
Ouput Impedance: 50ohm
Output harmonics: 40dB down on the
main carrier as required by WT regs
6. Modulator
Schematics
The schematics reflect the circuit
very closely. However there are no component references and
occasionally you may be able to find a value on the schematic
which is not correct according to the prototype. Also the op-amp
sections and hence pin numbers will not match. The schematics
are drawn using Windraft from IVEX design.
7. Modulator
Circuit Description
Cheap AM
Transmitter Frequency Source 7.1 MOD01-01.SCH
This sheet shows the frequency
synthesiser and modulator. The 74HC4060 uses a cheap 4.608MHz
crystal to develop the 9kHz reference required for the VCO. The
internationally agreed channel spacing in Europe on MW is 9kHz
so all other frequencies are available by fairly simple circuit
changes. The exact frequency of the output should be adjusted
with the 10pF trimmer only when the circuit is fully warmed up
and the crystal oven is operating normally. It is sufficient to
adjust this by measuring the final output of the system at
1602kHz. The signal should be 1602kHz +/- 2Hz, so a frequency
counter with seven or more digits is needed. The 74HC4046 has a
centre operating frequency of approximately 1602kHz and a range
of about 200kHz above and below this. Output from the VCO is
divided by 178 in the 74HC4059 to form the comparison signal to
feed into the phase comparator with the reference signal. If a
frequency other than 1602kHz is required, this can be obtained
by changing the setting of the divider. If a large change of
frequency is required (eg for use on 963 or 999kHz) the loop
filter and VCO free running components will also have to be
changed. The values for all the resistors and capacitors of the
VCO and reference oscillator can be found in the National
Semiconductor or Philips HCMOS databooks. The only unusual
component is the 100nF capacitor filtering pin 9 of the VCO.
This reduces 9kHz phase noise on the output signal. (audible
when tuned just off centre of the signal with an AM radio). The
10M resistor reduces phase noise also, though the mechanism by
which this is happening is not fully understood. A better loop
filter than the simple one used here is proposed for the next
design iteration, using a 2 pole active filter rather than the
100nF capacitor, with the other filter components being similar.
Schmitt triggers buffer the signal output and form an inverse of
it. The true and inverse 1602kHz square waves are fed into the
carrier inputs of the MC1496 modulator. The use of the true and
inverted signals operates the
modulator in a differential saturated mode. (See the Motorola or
National Semiconductor databooks) This avoids the need for
special balancing components and a constant amplitude carrier
signal source. The 1k pot on pins 2 and 3 on the modulator is
the only gain control in the RF section. This sets the RF output
level ultimately delivered to the PA via the coaxial cable. The
tuned transformer acts as a filter removing the higher order
harmonics from the signal. This should be peaked for maximum
output. Unbalanced audio is fed into the modulator. As the
desired output signal is AM, not SSB or DSB, exact balancing is
not important. The amount of residual carrier fed through with
no audio is set by the ratio of the two resistors on pins 1 and
4 of the modulator. The stability of the modulator and these
resistors is important as they effectively set the audio
sensitivity of the modulator. Care must be taken to avoid noise
and voltage dips on the 5V line as the VCO is sensitive to
supply line changes. To this end, a separate 5V regulator has
been included for the VCO alone, run from the 22V regulated
rail.
Crystal
Oven And Power Supply 7.2 MOD01-02.SCH
The PSU is straightforward and is
covered adequately in the National Semiconductor regulator
databook. The crystal oven heater control circuit uses an LM393
comparator and a Philips NTC thermistor. Points worth noting are
the thermistor which is glued onto the top of the crystal with
epoxy and the 120R heater resistors which surround the crystal
on two sides and are also glued to it. The circuit has
hysteresis provided by the comparator itself and the 2.2M
feedback. This means that the BD139 operates in saturated mode
except when switching, and the whole system is "stable", in so
much as it is guaranteed to oscillate about the temperature set
point reliably. Supply line glitches are minimised by the 47uF
capacitor on the base of the BD139 slowing down the transistor
switching time. Even so, a separate supply line and ground
return direct to the regulator must be used for the heater
current so that the VCO is not disturbed when the heater
switches on and off. This is not shown schematically. Though not
intended to be a deeply precision circuit, this should be
sufficient to keep the crystal at an approximate elevated
temperature of 30 or 40 degrees C, in order to ensure that the
output is kept to +/-2Hz. The 10K pot on the inverting input
sets the temperature which ought to be set with a therocouple or
similar attached to the crystal. In practice, I would set it
such that the heater comes on with about a 10% duty cycle at
room temperature. The RF output stage currently uses a Nat Semi
LM6361 shown. The output biasing diodes are glued to the metal
transistor cases. The 2.2pF feedback resistor rolls off the
output at frequencies much above 2MHz. This driver can give the
required 2Vpp into 50R even with the 47R series output resistor.
This resistor ensures that the output impedance is close to 50R
and should avoid problems even if the cable to the PA is badly
terminated. The 4.7K resistor on the RF line should not be any
higher than this value or else the transformer on sheet
MOD01-01.SCH exhibits too high a Q value and starts to filter
out the AM sidebands.
Gyrator
Filters Band Splitter 7.3 MOD01-03.SCH
This sheet shows the front end of the
audio processing. Throughout the audio sections, TL074s are
used. Though far from being a rocket science audio op-amp, it is
widely available, cheap, and intended for low-noise audio use.
The differential left and right stereo inputs are converted to
single ended and then summed together to form a mono signal. At
the summing point there is also a division of amplitude by two
for each signal. So if a zero dB tone is fed in phase to each
input, a zero dB tone will result at the ouput of the summing
amplifier. Three gyrator based filter stages follow. The Q of
these stages is set such that the filters are “two octave”
filters, ie the centre frequency of each filter is seperated by
two octaves and the outputs, if summed back together would give
a flat response. The design is lifted from a Maplin graphic
equaliser design, but a similar circuit is described in the
National Semiconductor op-amp databook. The three filters
separate the low, mid and high audio frequency bands which go
onto sheets MOD01-04.SCH and MOD01-05.SCH.
Split Band
Compressors 7.4 MOD01-04.SCH
Each of the low, mid and high sections
are taken to nearly identical compressor stages. The stages for
the low and mid bands are shown on MOD01-04.SCH. These are based
around the inexpensive Philips NE572 compandor. These devices
have several advantages over similar devices such as the
technically superior SSM2120 from Analog Devices.
1) They are a lot cheaper and more
widely available.
2) They work from a single supply -
the SSM2120 does not, without going to extreme lengths.
3) The NE572 has independent attack
and decay capacitors.
4) The NE572 does not have the pathological high frequency input
oscillation of the expensive Analogue Devices part.
The compressors are wired according to
the Philips databook, with the unity gain point being set at 0dB
for this application. This is more a matter of convenience for
testing than function. The compressors are wired in such a way
that they inherently produce 2:1 compression, ie a 2dB increase
in the input will produce only a 1dB increase in the ouput.
Producing ratios other than 2:1 1:2 or infinite (limiting/AGC
behaviour) with the NE572 is difficult, and so our compression
ratio is conveniently chosen for us. The attack capacitors are
the smallest that can be used without getting a significant loss
of level detection performance. As there are higher audio
frequencies involved, the attack capacitors on the higher
frequency sections have lower values.
100%
Modulation Limiter 7.5 MOD01-05.SCH
The output of each compressor section
is summed back together. This output is fed into an active
limiter stage consisting of the remaining NE572 section. The
threshold of the limiter, ie the point at which no more increase
in output is seen, is set to be 0dB nominally with the pot and
diode arrangement fed from the 5V rail. This may be a little
sensitive to temperature and voltage variations and may give a
slightly soggy limiting point. In the next design iteration an
op-amp fed from the NE572 voltage reference, and a diode drop
temperature compensation scheme will be adopted. This being
said, the simple circuit works perfectly well. The active
limiter will bring down large signals to 0dB, but there is a
delay before it can do this, due to the attack capacitor.
Because of this a passive diode limiter is used after the active
limiter to avoid overmodulation on transient audio peaks. When
setting up, the active limiter is put into limiting by applying
a +6dB audio signal to the inputs. The modulator drive pot is
adjusted for about 80% modulation. The diode limiter pot is then
adjusted such that the diode limiter is just starting to operate
at 80% modulation. This means that there is only transient
distortion of the signal by the diode limiter on speech peaks
etc and the modulation never exceeds 100%, despite the
modulation level being nearly 80% for much of the time during
music. The diode limiter is quite soft in operation, giving a
pleasantly progressive limiting action and producing minimal
audible distortion.
MW Standard
Low Pass Filter 7.6 MOD01-06.SCH
The processed audio is finally low
pass filtered prior to being passed to the modulator. The filter
is currently only six pole, and if this was the only form of
filtering it would only produce half of the roll-off required at
9kHz. The Radio Authority regulations on this point stipulate
that the modulated signal shall have at least -3db roll-off at
6.3 kHz and -40dB at 9kHz to avoid interference to stations on
adjacent channels. Some additional filtering is actually
provided in the RF tuned stages, where the bandpass elements
have the same effect as the low pass elements in the audio
stages, by reducing the level of the sidebands most distant from
the carrier frequency. As I have not tested the output with a
spectrum analyser yet, it may be necessary to put in more audio
filter sections here to fully comply with the regulations. It is
most unlikely that this system is any worse than the
transistorised LPF in the current transmitter, especially as
that has only an external active limiting system.
Nov 2002: Extra poles were added when I changed the carrier frequency output as mentioned previously.
8. PA
Schematics
There is only one sheet for the PA.
Though the schematic closely follows the prototype, there may be
a component substitution occasionally.
9. PA Circuit
Description
Remote MW PA TX01-01.SCH
The input signal from the modulator
unit, presumed to be at the end of a long piece of coaxial cable
enters the PA via an isolated BNC socket. The cable must be
terminated externally with a 50R load. If the same cable is
driving both PA sections, a T splitter must be used to connect
both inputs together. Note that due to the relatively high input
impedance of the PA inputs, one cable with a 50R teminator can
easily drive both PA sections. The signal is coupled into the
input transformer with a 1K resistor, ensuring that the input
impedance will be >1K and mostly resistive. The secondary of
the transformer is tuned and couples into the op-amp input. The
4K7 biasing resistor also damps the Q of this tuned circuit,
ensuring that the sidebands of the signal do not get attenuated
too much. The BD139/BD140 and op-amp combination is a standard
push-pull configuration which forms the PA driver. If possible
the base bias diodes should be glued to the transistors. The
signal from the driver is AC coupled to the gate of the PA
MOSFET. An LM317 is used to provide the gate bias for the
MOSFET. This should be initially set such that the PA current is
about 100mA with no signal input; this is generally around 4V.
The PA bias can then be fine tuned by injecting an 80% modulated
carrier from the modulator and adjusting the bias for maximum PA
linearity when operating near full power. The 0.25R resistor in
the source of the MOSFET provides some DC negative feedback to
stabilise the bias point. The AC negative feedback components
from drain to gate provide extra PA stability and tend to damp
VHF harmonics which can be created when the PA is overdriven, or
if the load is accidentally disconnected. The 10R gate resistor
is another damping component to improve stability. The stability
of the PA is very good, even when no load or a short is present
at the output. The overdrive characteristic is very good too,
with quite soft clipping appearing under overdrive conditions,
prior to the current limit operating. The ouput from the PA MOSFET is matched into the
output filter with a 1:4 transmission line transformer. The
output filter is taken from standard filter tables and rolls off
rapidly above 2MHz. If possible, future designs will use the
next size up of toroid core for the filter inductors, so that
thicker wire can be used.
The PSU is entirely standard, except
to note that it is possible to set the voltage operating point
of the PA such that the LM317T is providing its maximum 1.5A
just at the PA clipping point. In this way, if the PA is driven
beyond the clip point, more current will be drawn and the LM317T
will go into current limit shut down, discouraging further
advancement of the gain control. This is a good way to provide
the maximum possible peak ouput of the PA, while making it
difficult to abuse.
10. PA
Isolation
The 1:4 output impedance transformer
and the input transformer electrically isolate the PA ground
from the cable grounds. This breaks the loop which can be
created when studio mains earth is connected to F-block mains
earth and possibly any other block's mains earth if the coax
connectors touch pipework in the attics. Hopefully this will
reduce the amount of hum experienced both in the studio and when
listening to the signal. I have experienced a situation where in
certain orientaions an AM radio will produce a sound with a
large hum content, despite the transmitted signal appearing
clean. Rotating the radio will usually reduce it to a minimum,
but the isolation now provided should help eliminate this
problem. PA internal circuit ground is connected to mains earth
and the PA chassis.
There is an important safety aspect
here as well. In the F block technical cupboard you have F block
mains earth, possibly any other block's mains earth on the
transmitter cables and possibly studio mains earth or even
outside earth on the signal cable. As these could all vary in
potential by hundreds of volts, even without a fault, (and
believe me, they sometimes do!) you need to take care when in
the technical cupboard. For example, do not grasp the chassis
earth and the transmitter cable earth at the same time unless
you can help it. There is no easy way round this, as we
inevitably may get an ouside earth connection on the cables, and
even this can be a potential hazard if it is led into a building
with the modern PME earth system. The best way is to have all
the mixed up earths where they are in the technical cupboard,
safely out of harm's way. At least the PA isolation will stop
the mile-wide ground current loops.
11. Other Operational And Safety Issues
The user of the PA now has access to
10 Watts of RF power. It is essential that the far end of the
transmitter cables are terminated in 50R RF dummy loads capable
of dissipating the power that they receive. The loads must be
positioned or cased so that they are in absolutely no danger of
starting a fire. Such (supposedly 15 Watt) loads are available
from Maplin, but they do get quite warm and would need to be
fixed into a case with vents. A better solution would be to buy
a higher power load. The cable in general must now be kept in
good order all the time, to avoid the possibilty of hotspots or
high RF voltages appearing on the shield. When cable breaks
occurred previously, with 0.5Watts, the system just stopped
working. There is now safety to consider as well.
Do not be tempted to overdrive the PA.
The absolute maximum peak ouput is 100Vpp on modulation peaks.
Remember that if you are setting up the transmitter with a tone
it will only be about 80% modulated due to the active limiter,
so you must leave room for musical peaks. Though not terribly
technical I would recommend finally setting up the PA level,
when installed, using music and adjusting the front panel input
attenuator control until musical peaks just hit 80Vpp at the
output, assuming that you desire maximum power.
I don't recommend twiddling any of the
pots in either modulator or PA, even with the benefit of the
information provided here. When setting up, make sure that the
internal phone system and other radio services are free of the
signal. Radio Glen has been known to appear on the internal
phone system under certain conditions.
12. Update And Mistakes
OK, so the idea of isolating the
inputs and outputs on the PA wasn't such a great idea after all.
In fact when you consider that the output transformers are only
really DC isolating, all my great arguments start to fall to the
ground somewhat. The PA was found to go a little bit unstable in
the real system, characterised most noticeably by the gain
controls not working terribly well. The PA is in use at the
moment with the BNC plugs connected to the chassis with a piece
of wire which cures the instability for the moment. The best
course of action is to use non-isolated sockets for both input
and output, to bridge the isolation gap between the inputs and
outputs of the output transformer, and to bridge the isolation
gaps between the bolts and the rest of the copper plane on the
PA modules. This ought to consolidate all grounds to chassis and
the mains earth connection.
So this will take F-block mains earth
into the new technical cupboard, where it will be isolated from
earth there by the isolated output of the modulator.
I did once mention that the old
transmission system used to get hum on it sometimes, which was
not visible on the RF output. I associated with big ground
loops. Since this time I have considered the possibilty that
some of this was caused by 100Hz FM modulation of the AM signal
which was clearly audible when slightly off-tune but would not,
indeed, be visible on an oscilloscope trace.
13. Pictures
Inside the case
Inside the case with a close up of the PLL board and tin box with the oscillator inside.
Rear Panel - Left and right audio inputs and 50 Ohm Rf output
I don't think that any pictures of
the main transmitter PA exist any more.