Dedicated 198kHz TRF receiver.

[ukcol]

That's interesting.

After a quick and very careless glance I thought it was a superhet. "Why would they go to all the trouble of building a superhet for one station?" I thought.

They hadn't of course.

[Radio Wrangler]

There are many receivers around which are fixed-tuned to one frequency and the great majority of them are superhets. The need for a superhet arises when the transmission is not on a frequency which is well-suited to making filters appropriate to the wanted signal's bandwidth and the proximity of adjacent signals. I'm currently having a bit of a fight with a single-frequency receiver development - it has to meet a few awkward specifications.

Fixed-tuned receivers aren't common in the broadcast world where there is a choice of stations you might want to listen to. Outside of that world, there are plenty.

David

[Mike. Watterson]

The TA7642 (the cheap current version but different pinout) works fine Midwest of Ireland as a 198 kHz TRF. The "earthy" end of the tuning filter is the AGC feedback.
Using about 50 turns of wire on a 40cm frame made by two pieces of panel wood in a cross works better than a ferrite rod. Wind a wire loose at right angles around the coil connected one end only to TRF 0V as an electrostatic screen, about five turns around each side.
A diode only AM radio can only get RTE 1 252KHz during the day and needs a long wire.

[John_BS]

Having designed antennas for 198kHz, I would comment that for fixed-tuned work, you should be aware of the very high (positive) temperature coefficient exhibited by most "antenna" ferrite rods. You will need at least N750 tuning caps, with some N1500 thrown in if you can find them.
The typical Q of a well-made LF antenna will be a bit of a problem if you want to faithfully use the full broadcast bandwidth (not that it's very high in the first place!). For instance, a loaded Q of 130 will yield a response 12dB down at +/- 3kHz.
Regarding post 11, yes, 198kHz carries an LF radio-data service with +/- 22.5 degree phase-modulation / 25 Hz baud-rate. Droitwich also employs dynamic carrier control in order to save shed-loads of electricity, so the carrier power is reduced by up to 3dB during periods of heavy modulation.
John

[G0HZU_JMR]

Interesting to hear about the carrier control on 198kHz. I do remember seeing the carrier wobble a bit when I last looked at 198kHz on a spectrum analyser and I wondered why it did this. I recall that the typical RF BW on 198kHz is about 11kHz and it drops steeply after that so the AF limit will be about 5.5kHz.

This was received on a spectrum analyser with a tiny active antenna that has very wide bandwidth. It is just a 4" whip followed by an active stage that generates a negative impedance to get lots of bandwidth despite the tiny size. It generally works well over a range of a few kHz to about 750kHz so there is no chance of the antenna affecting the measured bandwidth of R4. I normally use this little active antenna as an E field wand around my workroom to sniff for interference from things like battery chargers or displays etc.

One other issue with ferrite materials is that they may generate intermodulation products with fairly large signals if they are used in a narrow BPF. This would normally only be an issue if the receiver was used with a classic longwire antenna. However, I can usually receive BBCR4 here at -10dBm using the little 4" active whip and this is a huge signal. But then again I'd guess that a lot of the basic BJT based receiver designs posted up so far would generate some intermodulation in the receiver section anyway. But probably not enough to be really noticed.

[G0HZU_JMR]

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

It might need some careful design if it is to support both modes with good performance. Many years ago I designed a quick and dirty 198kHz TRF receiver for use in an offair standard. I used a high Q ferrite rod antenna that has a bandwidth of just a few kHz followed by a JFET preamp and a very narrow BPF. This was then followed by a hard limiter IC to try and squish away the AM and just leave the carrier and the LF phase modulation.

This therefore uses a very different approach to a regular AM broadcast receiver because the bandwidth of the front end is very narrow and the receiver uses a fairly decent hard limiter. So there is no AGC and the limiter does a fairly good job of removing the modulation. I felt that I had to use this approach if the offair standard was to give a stable 10MHz output. So I'm not sure how successful it would be to try and design a receiver that does AM demodulation at full bandwidth with AGC and also has options for limiting and narrower filtering. It could be done but it would require some compromises or maybe some switched stages that suit each mode.

[turretslug]

Somewhere.... somewhere.... probably in the bitsa trunk in the loft is squirrelled away an early '60s technology 200kHz carrier receiver, a TRF with OC71-type devices and a large crystal filter and limiting amp. R4 may only be 2kHz away nowadays, but it's a yawning way away in crystal filter terms....

I'm a little worried that a TRF using a ferrite rod or "wickerwork" internal loop as found in '50s table radios could run into problems with the aerial being so close to detector/AF/speaker connections- the result being something more like a second-rate and anti-social Theremin than a useful radio, hence the suggestion of a screened IFT etc. can for RF input with remote aerial- the coupling could give scope for a band-pass input to avoid the sideband cutting highlighted earlier.

[ukcol]

Originally Posted by ukcol

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

I realize that the requirements for a receiver and those for a frequency standard are quite different.

Perhaps the approach would br to take the signal off at some convenient point and feed a narrow band 198kHz stage and limiter as described above. This signal could then be used to control a VCO perhaps.

198 kHz STANDARD is probably overstating the requirements for our purposes.
An accurate enough source could be built much more easily.

[ParcGwyn]

I built a single transistor 198kHz (actually 200kHz then) when I was a teenager, this provide loudspeaker reception. I thought it was called the Unity and was from a design in Radio Constructor. I have been unable to find the article on https://www.americanradiohistory.com/ however I came across a Sir Douglas Hall article on page 57 of https://www.americanradiohistory.com...RC-1963-08.pdf this may have been the design I built. Living in Malvern the reception was good.


Dave

GW7ONS

[Radio Wrangler]

Originally Posted by G0HZU_JMR

One other issue with ferrite materials is that they may generate intermodulation products with fairly large signals if they are used in a narrow BPF.

There is natural non-linearity in ferrite materials due to their B-H curve.

In a resonant circuit, the current in them is magnified by their Q factor (Marconi used to call Q the circuit magnification factor) so the driving field is also increased by the circuit Q.

It isn't just the antenna matching circuits at the transmitter which start rolling off the modulation from higher audio frequencies. The Q of a resonated ferrite rod at the receiver joins in. A Q of 100 will put the -3dB points 1.98 kHz apart, giving 3dB roll off on 990Hz audio. So the in-circuit Q needs to be kept low. Any other RF tuned circuits need similar management of Q.

That Armstrong tuner with an up-converter for long/medium wave reception dodged this problem, but its mixer needed to survive being clobbered by everything at once.

David