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Old 29th Nov 2018, 8:31 pm   #1
ukcol
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Default Dedicated 198kHz TRF receiver.

As mentioned in this thread

https://www.vintage-radio.net/forum/...84#post1096684

I had planned to build a high quality 198kHz only TRF receiver.

The idea was to eliminate the problems associated with the TRF (those of stability and selectivity) and also those associated with the superhet in one project and use it to listen to about the only station that is still worth listening to on AM radio.

The key to the project is the 198kHz. It will be possible to tailor the bandpass response to exact requirements and not suffer the image problems associated with low intermediate frequency superhets.

The set could also be used as a frequency standard to check or adjust the clock frequency in frequency counter equipment.

The project is not going to happen now but I have put the idea forward for comments from you gentlemen and ladies and perhaps even one or two of you may even consider having an attempt at it yourselves.

Last edited by ukcol; 29th Nov 2018 at 8:38 pm.
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Old 29th Nov 2018, 8:51 pm   #2
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Default Re: Dedicated 198kHz TRF receiver.

Could this be a job for a ZN414 (or its later plastic-encapsulated descendent whose number temporarily escapes me?) I remember they're not really designed for such low frequencies (the internal coupling capacitors are a bit small) but if you've got a strong-enough 198KHz signal you would probably not notice the odd 10sB of loss.

If you're using a ferrite rod antenna or a pot-core type coil you may hit the issue of the 'Q' at resonance being so high that it causes some attenuation of the sidebands and you get an unexpected degree of top-cut.
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Old 30th Nov 2018, 12:50 am   #3
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Default Re: Dedicated 198kHz TRF receiver.

It will probably make the purists wince and turn away in horror- on several counts!- but a quick and lazy way of getting a 198kHz TRF strip might be by cannabilising a couple of '50s Cinderella woodies, the sort of thing that no-one bids for at one of the vintage events and re-resonating their 465kHz-ish IFTs to 198kHz by making the original capacitors up to 5.5 times their original value. The coupling will be different and maybe wrong but I daresay 3 IFTs and a couple of (say) EF92s in what would otherwise be a textbook 4 valve circuit from detector/AGC on could be tuned up for a fidelity bandwidth and good sensitivity. Aerial coupling to 1st ex-IFT subject to various approaches, say capacitive tapping-down at low impedance for a loop coupling link, or top-coupling for a high impedance wire. Alternatively, substitute "'60s/70s workaday trannies" and "3x AF117 etc" in the above scheme.

I'm almost tempted to try it myself, but by tomorrow I may have spotted the glaring, show-stopper flaw in the above reasoning....
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Old 30th Nov 2018, 5:27 am   #4
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Default Re: Dedicated 198kHz TRF receiver.

About 30 years back I used a surplus Advance OFS2B standard to make a receiver (it's
a TRF design) by using a simple diode demodulator and LM386 audio amplifier.

We've been here before
https://www.vintage-radio.net/forum/...ad.php?t=23042
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Old 30th Nov 2018, 8:51 am   #5
Karen O
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Default Re: Dedicated 198kHz TRF receiver.

I have a modern version of the ZN414 working on LW.

It works fine, though I'm only 25 miles or so from Droitwich so I may have an advantage.
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Old 30th Nov 2018, 10:21 am   #6
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Default Re: Dedicated 198kHz TRF receiver.

A TRF suffers from feedback when high gain is used. This can be controlled with a semiconductor circuit but would be very difficult with valves.
I would go for a low Q tuned input circuit, then an amplifier feeding a 20kHz bandpass filter, then a detector with AGC (ZN414, TA7642). Each stage should be placed in a sealed box.
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Old 30th Nov 2018, 11:52 am   #7
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Default Re: Dedicated 198kHz TRF receiver.

There was an interesting simple 2 transistor TRF tuner design for (then) 200kHz in the Radio Constructor 'Dick & Smithy' column in 1972: http://vintageradio.me.uk/radconnav/...72/circuit.jpg

The problem with using 198kHz as a quality source is the brickwall filters used to keep the transmitter in band, and the frequent transmission faults.
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Old 30th Nov 2018, 12:21 pm   #8
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Default Re: Dedicated 198kHz TRF receiver.

Would an SDR be considered to be a TRF if it used a tuned loop as the antenna? Nice easy brick wall filters.
 
Old 30th Nov 2018, 12:41 pm   #9
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Default Re: Dedicated 198kHz TRF receiver.

I have a PCB from an old Economy-7 meter that has a circuit tuned to 198kHz (or thereabouts). Seems like a reliable receiver given its application - I'll dig it out to see what the main components are.
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Old 30th Nov 2018, 3:46 pm   #10
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Default Re: Dedicated 198kHz TRF receiver.

As well as the Smithy & Dick 2-transistor circuit (see above) in the Radio Constructor, there's a 'Suggested Circuit' (by G. A. French) which uses a 'twin T' circuit (again, 2 transistors, so 2 stages, I think) to tune 198kHz.


I recall making up the circuit when it came out, but not having any success. That's probably down to my construction, rather than the idea, though.


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Old 30th Nov 2018, 3:47 pm   #11
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Default Re: Dedicated 198kHz TRF receiver.

Quote:
Originally Posted by ukcol View Post
The set could also be used as a frequency standard to check or adjust the clock frequency in frequency counter equipment.
I may be wrong, but is not the 198Khz Droitwich signal very narrowly frequency modulated to provide some commercial communication service or other? Tony.
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Old 30th Nov 2018, 3:50 pm   #12
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Default Re: Dedicated 198kHz TRF receiver.

The idea of re-resonating a 465KHz IF-strip is a goodie: I remember that back in the 1960s there was a "Practical Wireless" fixed-tuned-to-Luxembourg tuner project that used the same idea but re-resonating a surplus 1.6MHz IF-strip to 1.44MHz/208-metres.
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Old 30th Nov 2018, 4:51 pm   #13
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Default Re: Dedicated 198kHz TRF receiver.

Quote:
Originally Posted by TrevorG3VLF View Post
A TRF suffers from feedback when high gain is used. This can be controlled with a semiconductor circuit but would be very difficult with valves.
Although the above statement is true for the TRF receiver in general, the design of a single fixed frequency 198kHz amplifier should not present any extra problems over the design of a 120kHz or 470 kHz intermediate frequency amplifier. If fact 198 kHz is nearer to the lower end of that range where stability, gain and bandpass requirements are easier to achieve.

Low intermediate frequency transformers (from a donor set) could be used with some turns removed and some extra capacitance added. Having the equipment to measure inductance accurately would be an advantage if like me you have one of the PEAK instruments and/or a good vintage bridge. I am sure one of our many contributors would assist with the calculations required to tailor the bandwidth to the BBC R4 transmission specification.

My bucket list receiver would have been a valve one but a semiconductor one is generally easier and cheaper to build of course.

I'm looking forward to seeing your designs and builds so get to work.
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Old 30th Nov 2018, 5:37 pm   #14
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Default Re: Dedicated 198kHz TRF receiver.

All of my past problems with howling radios has been IF feed-through to the AF amplifier, and emission from the speaker leads. With adequate filtering on the speaker leads it should be okay (this is where transformer outputs gave that for free).
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Old 30th Nov 2018, 8:08 pm   #15
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Default Re: Dedicated 198kHz TRF receiver.

I have a circuit for 198 kHz tuner from PW (or possibly PE) published in the mid-late 70s. It used BF224 FET in the front end. From memory it used a ferrite rod and the load for the FET was an IF transformer with extra capacitor.

Think I've found it; PW July 77 "Radio Two tuner for your Hi-Fi" p213. Link here: https://www.americanradiohistory.com...PW-1977-07.pdf

I made it and it still works.

Ian
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Old 30th Nov 2018, 9:01 pm   #16
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Default Re: Dedicated 198kHz TRF receiver.

Quote:
Originally Posted by merlinmaxwell View Post
Would an SDR be considered to be a TRF if it used a tuned loop as the antenna? Nice easy brick wall filters.
Probably not. Bandpass filters are very inefficient in DSP, so the usual ploy is immediately the digitised signal is got from the ADC it is downconverted in a pair of multiplications fed by quadrature 'oscillators'. This gives a quadrature pair of signals centred on zero frequency, which are then processed with a pair of low frequency lowpass filters. These define the receiver bandwidth, reject adjacent channels etc. etc.nFor AM demodulation the two signal paths are squared, added together and square-rooted to get the scalar amplitude by Pythagoras' theorem.

So the processing signal flow is essentially that of a direct conversion receiver which is technically a superhet.

But is it of any importance?

You can consider that a normal TRF with a diode detector is also a direct conversion superhet. There is no local oscillator, one is not needed because there is a distant one, the incoming carrier drives the detector into significant non-linearity, and thereby intermodulates with the sidebands, one of the products being the demodulated audio baseband. Job done!

Funny how they all come down to essentially the same thing.

David
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Old 30th Nov 2018, 9:19 pm   #17
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Default Re: Dedicated 198kHz TRF receiver.

You can make wide enough filters to encompass a good wide AM transmission at 198kHz and lower. The difference is that simple synchronously peaked stages will no longer do the job and that something like a Butterworth of Chebyshev pole pattern is needed. This isn't any more demanding in terms of numbers of parts or numbers of stages, but maths is involved in the design, not just blind peaking at a spot frequency.

The good old Racal RA17 has a 13kHz filter at its 100kHz IF.

As a *very* rough guide for LC filters, you use coupled-resonator styled where the bandwidth is less than 10% of the centre frequency, and the lowpass to bandpass transformation of a lowpass prototype when the bandwidth is greater than 10%. In practice there is a lot of overlap. Make a coupled resonator much wider and the skirts go asymmetric, though there are ways to handle this. Make a transformed-lowpass filter too narrow and the design calls for ludicrous component values considering the centre frequency and demands impossible stray values.

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Old 1st Dec 2018, 1:03 am   #18
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Default Re: Dedicated 198kHz TRF receiver.

Hi.

There was also an interesting circuit by Sir Douglas Hall entitled The 'Droitwich' car radio. This was a hybrid design using a ECH83 valve and three transistors. It used a couple of range 1 Denco coils. The project was in Radio Constructor, November 1971 and can be found here https://www.americanradiohistory.com...RC-1971-11.pdf It was obviously intended for the old 1500m (200kHz) carrier but would no doubt be easy to retune to 198kHz.

Regards
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Old 1st Dec 2018, 4:00 am   #19
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Default Re: Dedicated 198kHz TRF receiver.

In respect of valved TRF circuits, there were some in Wireless World during the 1950s, some variably tuned, some switch-tunes, but most probably amenable to adaptation to single-station form. Here is a partial list:

WW 1952 February, p.68ff: Amos - Midget Three-Valve A.C. Mains Receiver

WW 1951 November, p.452ff: Amos & Johnstone - Sensitive T.R.F. Receiver

WW 1952 October, p.406ff: Winder - “No Compromise” R.F. Tuner

WW 1953 May, p.233ff: Styles - Sensitive Two-Valve Receiver

WW 1954 April, p.165: Osbourne - Midget Sensitive T.R.F. Receiver

WW 1954 November, p.542ff: Amos - Miniature Bedside Receiver

WW 1955 December, p.616ff: Styles - Sensitive Three-Valve T.R.F. Receiver


From the solid-state era, I imagine that the relatively simple Herbert homodyne (WW 1973 September, p.416ff) would be quite amenable to fixed frequency use. Similarly the more complex Slifkin & Dori homodyne circuit. This was part of the Slifkin & Dori Synchrodyne-Homodyne described in WW 1998 November p.947ff.

The Linsley Hood Quality AM radio (WW 1986 September, p.16ff) might be adaptable to fixed-frequency TRF form by deletion of the mixer and oscillator, then using the two-stage IF amplifier as a two-stage RF amplifier. This circuit had a low-distortion demodulator and what looks to have been a better-than-average AF amplifier. Alternatively the combination of the demodulator and AF amplifier from this design with the two-stage TRF amplifier from the Linsley Hood Synchrodyne would probably work. The rather complex synchrodyne was described in three WW articles, 1986 January p.51ff, 1986 February p.52ff and 1986 March p.58ff. The TRF amplifier was described in the third article, p.60.

I am not sure what RF bandwidth would be right for BBC 198 kHz reception, but ±6 kHz might be a reasonable estimate. As far as I know the Droitwich transmitter was historically limited by aerial Q to somewhat less than the ±10 kHz that was once the de facto the norm for AM broadcasting, but there might also be AF brickwall filtering as was done with MF transmitters from c.1973.


The above-mentioned Wireless World articles are available (except 1998) at: https://www.americanradiohistory.com...d_Magazine.htm.



Cheers,

Last edited by Synchrodyne; 1st Dec 2018 at 4:16 am.
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Old 1st Dec 2018, 11:55 am   #20
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Default Re: Dedicated 198kHz TRF receiver.

Circuit diagram of the Bush TRP45."Top Ten" record player.This TRF receiver was fixed tuned to the 200Kc/s BBC light program transmitter.
More information can be found in the 1960/61 Radio and Television servicing book.

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