Frequency coverage is from 5,7MHz up to 22,5MHz. That might seem a lot, but with a 10 turn tuning pot it is quite acceptable. The foot print is 10 x 15 cm.
AF board on the left, audio pot and 10 turn tuning pot center front, and pre selector and oscillator coils on the right hand side. In this test setup I have separate batteries for the tuning voltage, the receiver and frequency display module.
A closer look at the coils:
Schematics: the TDA1072 is as shown in the data sheet.
Pre selector circuit:
Audio amplifier, schematics and frequency response (voltage amplification factor as function of frequency). The values shown are suitable for approximately 50mV plus/minus input signals.
The TDA1072 radio chip has an output for frequency counter which works perfectly with a cheap counter module found on ebay. The counter module has programmable IF offset which makes it easy to display receive frequency.
All in all I really like the TDA1072. It makes building a fairly good receiver doable. Using varicap diodes for tuning is very flexible and does away with a lot of the mechanicel headaches that comes with traditional variable capasitors.
Regarding my earlier comment about a regenerative IF for phase-locking to the incoming signal:
Just in case anyone is interested, the only remaining description of this receiver can be found here: http://www.vk2zay.net/comment/236 . Quote:
I did some experiments with this 10 years ago and abandoned the idea as I didn't know enough about superhets at the time. Might be interesting to return to the idea, now that I have some experience in successfully building a stable IF strip with transistors.
As I recall, the Pierce oscillator configuration used by the above author, and confirmed in hardware also by myself, was a very simple 2N3904 circuit with the emitter grounded, the crystal between base and collector, a low-ish collector load resistor (4.7k?), and some simple base biasing (maybe 100k to Vcc?), with supply voltage being used to smoothly control regeneration, which I could confirm by monitoring the radiated signal on a nearby receiver and hearing how the signal smoothly faded into the noise floor as the regeneration (supply voltage) was adjusted. On the (now deleted) original TRB forum, the original author also mentioned using some additional resistance in series (or perhaps it was in parallel?) with the crystal to lower its Q and widen the bandwidth for AM reception.
At the time, I wasn't successful in using the Pierce-based regenerative detector as part of a receiver, because I had insuffucient IF gain in front of it during my experiments.
EDIT: After a little digging around, I was able to recover the original discussion threads from the old TRB forums. I am attaching them here. The original author of the circuit, DXer, stated:
@coildog By the way, what kind of antenna are you using with this circuit? How does the circuit work when you use a very short whip antenna (say 30 cm or so)? I'm always interested in performance with very short antennas because that gives an idea of how much gain the receiver is providing. If the receiver noise level goes up when connecting even a very short whip antenna, then that's a good sign of sufficient gain.
As an aside, I have found that those PKJ type frequency displays tend to put noise back into the radio when connected to a common power supply.
FIx is easy - a simple RC network on the power supply line to the counter. 22R and 10nF will probably do it. Some trial and error might be required. I use the 6 digit version on my FM radio.
I put the manual for this type of counter to this post in case you don't have one.
Thank you for your kind comments!
Yes, when I build roughly the same circuits several times, it sometimes occurs to me that I could make small improvements. The thick copper wire (2,5 sqr mm - 1,75mm diameter) is a way to avoid too much chassis work. If I mount the receiver into some sort of cabinet, I thought I might find a way to let the knobs stick out of the front somehow.
Regarding the small shield between the LO and ANT coils; I think they are quite close in frequency operating at 15 - 20 MHz and the oscillator coil might induce some LO signal into the antenna port. I don't know if that is a problem, but I thought it might be a good idea to reduce the chance of it happening. It also stiffens the circuit board a bit.
Operating the LO at such high frequencies is a bit of a stretch. It is not super stable, it drifts around a bit, a couple of kHz up and down. Using phenolic (pertinax) circuit board material was also probably not a good choice. I guess I should have gone for fibre glass.
It would be very interesting to try some sort of PLL to lock the LO frequency. I have sort of been looking for a simple circuit to try but so far I have not found any. Maybe there is something for an Arduino project?
While I rember ...
Our friend W5JAG commented in another thread that the problems I was experiencing with the ceramic filter might be related to mismatched input and output impedances.
That appears to be right on the money ... I realized that I may have been ''overcompensating'' by having a 2k2 resistor in series with the input and a 2k2 resistor in parallell with the output.
In this circuit I have omitted the resistors and so far I think the filter is more accurate at 455kHz and it also sounds better. I may have to listen longer, and under different reception condition to make a definitive judgement on that though.