Pulse Counting FM Receiver

[라디오]

                                      The Pulse Counting FM Receiver

   Most of you would never have heard of this type of receiver, unless you're into top end FM tuners, or you have read British electronics magazines from the mid 50's to the 1970's.
   I first saw the design in a mid 1960's Practical Wireless magazine and due to the simplicity I thought it wouldn't work. How wrong I was! This was in 1990 when I was getting frustrated with the limitations of super regenerative receivers and was looking for other approaches to the 'simple FM receiver' problem.

First Pulse Counting FM Receiver, Winter 1990.

   I had the basic receiver assembled in one evening, and due to my scepticism, I'd added an extra IF stage in case the gain turned out to be next to nothing. Well, I was in a state of amazement when I first powered up my receiver and was getting Hi Fi sound with excellent sensitivity. At last a solution had been found! The following night I'd added a grounded grid RF stage to eliminate the problems of aerial coupling and absorbtion effects. With an indoor TV aerial I had no problems in bringing in all local stations and the low power community broadcasters that caused so much difficulty on the super regen sets. And there was no SCA/stereo subcarrier beat, no hiss, and no distortion!!
  Subsequent experiments proved that stereo reception was not practical. It appeared that the demodulated signal is not of sufficent bandwidth, and to increase it would mean reducing the already low output. However, with high frequency boost prior to the LM1310 stereo decoder, it was possible to get results of some sort but separation was just too poor.
  So what is the Pulse Counting receiver? Basically it is a superhet but it's after the limiter stage that things become different. Instead of feeding a ratio detector, phase detector, quadrature detector etc. the clipped waveform is differentiated and applied to a pulse counting, or tachometer, circuit. The higher the frequency, the closer the pulses are together, and the the higher the resulting DC from the detector, and vice versa as the frequency decreases. So, we have a frequency to voltage converter which is what we want. The greatest advantage is there is no alignment, and therefore no tuned circuits to get out of alignment, causing distortion.
  The other main advantage is that this type of detector is suited to a low IF; typically 150-300KC/s.
This means the IF stage can consist of resistance coupled amplifiers. No coils to wind and align!!
  The low IF lends itself to yet another advantage; the VHF to IF frequency converter stage needs only one coil! Because the difference between the received signal and the local oscillator is so small the one coil can perform for local oscillator and aerial, using an Autodyne converter.
  Commercially made Pulse Counting receivers are a bit more elaborate than this. They're often dual conversion, with a conventional 10.7Mc/s IF which is then converted down to the ~150Kc/s IF with a crystal locked converter. This is done to prevent image response, and also the avoid drift problems. With such a low IF, the local oscillator operating at 100 odd megacycles has to drift ever so slightly to lose the channel. These days they often have a digital type detector which provides good noise immunity. By passing the squared waveform through a Schmitt trigger, most of the noise will be removed.
  However, for the homemade type of receiver, we don't need to go to such lengths to get a good quality receiver.
  I have built four such receivers up to the present time and all worked first time, with nothing to align. The design is not 'weird' or a 'fluke'. Sensitivity is close to and better than some commercially made superhets. Yes, there is an extra control to be adjusted which is a tradeoff, and tuning can be more critical but for the person who wants high quality mono FM, using valves, with simplicity, there isn't a better receiver.
  The earliest mention of pulse counting techniques appears to be in the early 1940's where such a receiver was used to monitor one of the first FM transmitters on the Empire State building. It used 807's for the RC coupled IF stage (a bit of overkill) and a 6H6 detector.
  The next mention appears in Wireless World in the late 1940's when Thomas Roddam asks, "Why align FM discriminators?" and puts forward a pulse counting detector design. Then in 1956, M.G. Scoggie answers the question with a complete and practical Pulse Counting receiver. It was this design that provided the basis for subsequent valve receiver circuits until solid state versions appeared.
   My circuits have used the same IF strip (in some cases altered somewhat) and detector, as these shouldn't be changed unless you're going to take the time to do a response plot of the finished article.
   I have found best performance, coincident with ease of construction, results from using a 6BL8 at the front end. The triode is used as a grounded grid RF amp, feeding the pentode functioning as an Autodyne converter. Having such a low IF means that the signal can be tuned in with the local oscillator either side of the carrier. This can be advantageous if there's an interfering station on a nearby frequency.
   The original Wireless World article used a 12AT7 local oscillator driving a passive germanium diode for frequency conversion but the gain is obviously lower than an active first detector.
  A two stage IF amplifier was used using 6BX6 valves with the response as shown:

Gain is about 4000, and input voltage (to the IF) should be at least 1mV for optimum performance of the following limiter and detector. In reality, with the converter gain, the receiver works well at signal levels much less than this.
So how do we get this response with no coils? It's quite easy actually when you consider things like valve capacitances. Note that each of the 6BX6's has a 4.7K resistor in series with the grid. In conjunction with the grid capacitance there is a degree of low pass filtering. The high value of plate resistors (18K), adds to this and so we have the top frequency response set to around 250Kc/s.
The low frequency response is set by the 270pF grid coupling condensers in conjunction with the 100K grid resistors. It starts to fall off at about 20Kc/s.
Here is the circuit of my 4th receiver which illustrates the points under dicussion:

Circuit of my 4th and latest receiver. Click here for full size circuit.

Limiting is done in the conventional way with a third 6BX6 operating with low bias and a low value plate load. Because of the low frequency used, it is quite easy to observe the waveform with a CRO throughout the IF, limiting and detection stages.
Despite it's appearance as a voltage doubling AM detector, the circuit around the 6AL5 is anything but. The low value input condenser (47pf) and the low value load (4.7K) ensure the signal from the limiter is differentiated. Filtering is done with a simple RC circuit which also provides de-emphasis.
Audio output is low at about 50-100mV and the recommended load is 500K. An ordinary triode pentode audio amp can be barely fully driven; an extra stage of gain is worthwhile.
 

Second Pulse Counting Receiver, October 1992. This uses a 6SN7 audio output. The resulting 'triode sound' is the nicest sounding of all my FM receivers.

                                  My 4th Pulse Counting Receiver

  Constructed in the Winter of 2000, this is my latest Pulse Counting FM Receiver. It is also the most straightforward of my pulse counting designs, and uses M.G. Scroggie's IF amplifier circuit, so this is the receiver I'll discuss here.
  This receiver was built to match an existing two valve MW receiver that I'd built in 1997, and was to use the same instrument case and control layout. I also wanted a smaller, more compact version of the receiver which I could take travelling.
   As previously, I stuck to my tried and trusted 6BL8 RF/converter stage, as I've found from experimentation that other valves or circuit configurations don't produce as good results.
   If you've seen the 12AT7 receiver article, you'll notice my RF amp design there. It's a grounded grid
circuit with input and output untuned to save winding coils and their alignment hassles. There isn't really any gain but it does have excellent isolation, which is the main purpose. As with the super regen sets, aerial loading can cause oscillation to become unreliable at certain parts of the band, or with the aerial in certain positions, etc.
  The frequency converter uses a 6BL8 pentode operating as an electron coupled oscillator, just like in the 12AT7 receiver. I have found this type of oscillator to be the simplest and easiest to get going of all the VHF oscillators I've tried, so I use it in all my valve VHF work.  As you can see, the screen voltage is made adjustable by the 50K pot. This is necessary to set the optimum operating conditions for the converter, so it oscillates reliably and provides maximum gain. Of course, the optimum setting varies from one end of the band to the other. So, despite being a superhet, there's still an extra control.
Maximum sensitivity occurs just after the converter has gone into oscillation. However, if you are content with only the higher power stations, just set the screen grid voltage at the 108Mc/s end of the band and leave it. Sensitivity will drop off as you tune to the 88Mc/s end, but the performance will still be better than any super regen set. A vernier dial is highly recommended for the tuning control, although a large knob is adequate. The dial I used on this receiver is one of the last from DSE, but you can still get them from Ocean State Electronics. As usual, the tuned circuit consists of a 15pF variable condenser and 4 turns of 18B&S TCW on an air cored 3/8" former.
   The IF appears across the 22K plate resistor and thence is amplified by the two 6BX6's. Limiting is acheived by a third 6BX6, whereupon the clipped output is fed to the pulse counting detector, a 6AL5. The IF, limiter, and detector is exactly as per the Wireless World design, except I've used a valve detector instead of germanium diodes.
   My last three Pulse Counting Reveivers have incorportated AGC. Without it, strong signals can actually overload the front end to the point where nothing is heard on that station. While placing an attenuator in the aerial lead solves the problem, overall sensitivity is reduced slightly. An ideal source of AGC voltage is obtainable from the limiter grid. Due to grid rectification, the voltage here becomes increasingly negative with an increase in signal strength. This is fed to the RF amp grid via a suitable voltage divider to reduce the gain sufficiently.
  Although I have not done much work with it, the limiter grid appears to be also suitable for providing an AFC voltage. With the receiver correctly tuned, the voltage is at it's maximum negative. Early experiments on my first receiver, with a varicap diode connected to the oscillator coil via a low value condenser, seemed to indicate the idea works.
  One could also add a magic eye at this point to indicate correct tuning.
 
 

Front panel controls: On/Off, Headphones, Volume, Tuning, Oscillation.

   Turning now to the audio amp, I've used another 6BL8. This is probably the most common TV valve in Australia. Europeans know it better as the ECF80 or it's series heater version, the PCF80 or 9A8. Initially, I used a 6DX8/ECL84, but the severe heat problems I discovered when I completed the receiver meant I had to do everything I could to reduce heat, and decreasing the power of the audio amp was one of the steps I had to take. Output power is about 250mW.
   The power supply is unusual as far as Australian design goes, using a live chassis with a transformer for the heaters only. This was done for space reasons. There was no way any valve type power transformer capable of powering this circuit could fit in the box with everything else. I used a Ferguson low profile PL15/20VA transformer. This has two 7.5V windings, so I paralleled them and ran the heaters via a low value dropping resistor to obtain the correct 6.3V. The 240V mains is rectified by a 1N4007 and smoothed with RC filtering. No hum is evident even with sensitive headphones. The audio amp and front end both run off about 200V.

Live Chassis Considerations:

Very compact inside with 6 valves! On the left are the front end valves. The bottom row is the 6BX6's, and the 6BL8 converter and 6AL5 are on the top. Over at the right is the 6BL8 audio valve. Notice the teflon control shaft extensions. This receiver has a live chassis.

   While my method of using the mains to provide the B+ directly enabled me to use the size of instument case I did, there were some important things to take into consideration. Because of this,
I don't recommend anyone building this receiver using the live chassis power supply; it is presented here purely as a description of my latest receiver.
   It is essential that the user cannot come into contact with the mains, and given that the chassis and internal circuitry is connected directly to 240V, the construction practices have to be altered. Using a plastic case solves most of the problem, but there's still other things that need to be done:
Control shafts: Obviously as the pots and tuning condenser are mounted on the chassis, their shafts could be live. Using plastic knobs is not the solution as knobs do come off, and the grubscrews can be accessed. Also in the case of the vernier dial, the reduction mechanism and dial is not isolated from the control shaft it's driving. The correct way is to isolate all the controls by plastic extension shafts.
Aerial connection: Here an user contactable electrical connection has to be made to the live circuit.
As with TV practice, I've used 470pF isolating condensers. The reactance at 50c/s means that less than 1mA can flow to earth if the chassis is live, but at VHF the reactance is considerably less, so the FM signal is passed into the RF stage relatively unimpeded. It is absolutely vital that the right condensers are used!! The correct ones are rated at 400VAC and are specifically for aerial isolation in TV sets. Use any others and you run the risk of breakdown and electrocution.
  Due to the capacitive coupling, it is possible for the aerial to acquire a static charge. This can cause breakdown of the isolating condensers. So, a high resistance path to earth (via the mains) is provided by two series connected 2.2M resistors. Two are used to increase the voltage rating. The centre tap of the balun is actually connected to the mains earth to provide protection against condenser breakdown and also for aerial static discharge. However, we cannot always guarantee the receiver will be plugged into an earthed supply (eg; two wire extension lead), so we still take precautions using high quality isolating condensers and the static discharge resistors.
Headphone/Speaker connection: The possibility of headphones becoming live at 240V should encourage any potential constructor to do the right thing here. Firstly, the output transformer has the primary and secondary wound on separate nylon bobbins. It is a conventional double insulated 240V to 6V 300mA power transformer. With an 8 ohm load, the reflected impedance is about 12K, which suits the 6BL8 output valve. Conventional speaker transformers are unsuitable as they are not mains rated! The low plate current does not cause noticeable ill effects due to core saturation. Negative feedback circuitry remains on the primary side of the transformer, allowing the secondary to remain isolated.
  The headphone socket was mounted inside a plastic shroud so that any parts coming adrift inside the case cannot touch the terminals. Wiring to the socket was enclosed in plastic tuning, thus double insulating it from the chassis. As a final precaution, one side of the headphone/speaker connection is connected to the mains earth.

Powering up the new receiver

  As with previous pulse counting receivers, this one brought in stations straight away. I did have to do a minor adjustment of the tuning circuit but nothing that required test instruments. The audio amp required a few minor component alterations to get the correct voltages, and likewise the converter screen grid or 'oscillation' control needed to have its voltage divider resistors optimised. Sensitivity was good as usual, with 2ONE on 96.1Mc/s at 5KW, receivable from 100km away with no aerial (Wentworth Falls > Gore Hill).
  However, it was becoming apparent that the heat build up inside the box with the lid on was a real problem. With the lid too hot to touch and the internal components subjected to such heat there was a risk of failure, as well as the chassis coming loose from the plastic studs securing it to the box.
To try and cool the inside of the cabinet as much as possible, I put ventilation holes at the rear and underneath the case. Furthermore, the heater and B+ dropper resistors were moved to outside the box inside a perforated metal shield mounted on the back. This was connected to mains earth of course.
This helped, but I was able to improve things even more. I replaced the 6DX8 audio amp with the 6BL8. This lessened the heater current nearly 300mA, so the transformer ran cooler. Also, the lower plate current of the 6BL8 lessened the heat generation.
  Testing at home in the Blue Mountains gave the expected results. I was able to receive a tourist information station in the Southern Highlands with my 5 element Exastereo outdoor FM aerial. These stations only transmit with a couple of watts, so it was a good indication of the excellent sensitivity.
  One thing that was becoming obvious, and annoying, was the dropping out of the oscillator at the high end of the band. I eventually discovered it was due to the 1.5mH cathode choke. For some reason the one I'd used wouldn't allow proper oscillation. Changing to a different type fixed that completely.
  For those wondering about frequency drift; it is only apparent in the first few minutes of receiver operation and is slight.

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