2007-09-16
Tone Dipper
This project was inspired by dipper related chatter on the regenrx Regenerative Receiver Yahoo group. Various designs were posted including these two which caught my eye: second last on this page, the Elektor one and also in German. The RF oscillator is the same emitter-coupled differential one as I use in the Noisy Regen and deserves more research. Mike AA1TJ suggests lower LC ratios are the key to getting good performance, but I'll need to build some more test equipment to experiment with this properly.
The idea is hardly new. It is even hinted at in EMRFD; there is a volt-to-tone circuit on 7.24, figure 7.46 which was intended for bridge nulling (It is quite elegant actually, being biased nicely). It has been a long-time ambition of mine to build a field strength meter/wave meter and dipper with audio read out. It simply isn't always possible to have your eyes on the meter while you are using such instruments.
So I built mine with a circuit essentially identical. I used a Jaycar potting box for the enclosure, a polyvaricon for the tuning capacitor and brought out both gangs and the oscillator connections to the coils through a 180 degree 5-pin DIN socket to facilitate easy coil construction and swapping. My only major break with the original design was the use of a piezo speaker and its connection across the AF oscillator collectors and the addition of 1 nF decoupling capacitors.
I used BF199 devices for the RF oscillator, a 1N5711 for the diode and a pair of 2N3904 for the AF side. The power switch was implemented by requiring the plug in coils to short the centre pin to the decoupled collector of the RF oscillator. Before placement in the enclosure (and extra lead length) the oscillator topped out at over 480 MHz with a hairpin inductor and no tuning capacitance, the DC being supplied in the middle of the hairpin with a 100 pF decoupling cap to the ground plane. Once in the box the strays limit operation to about 250 MHz. More careful construction (i.e. fixed on UHF with one coil or much better plugs) would allow low UHF operation with these transistors.
The tone shift isn't exactly spectacular, more work on the AF side of things would be a good idea. However, the device works quite well, even the most tone deaf should be able to use it. You can watch a short video(8.98 MiB) of me dipping the self-resonant frequency of a Ohmite 10 uH RFC. It is specified for 65 MHz, and it dipped at 67.9 MHz just laying on my desk. It easily dips my FM radio antenna and with various coils can be used on HF. It does seem to work better the higher up you use it, I'd say its optimal on VHF. On MF the frequency shift wasn't really sufficient for clear dipping, some improvement might be had by varying the emitter resistor in the oscillator to reduce the negative resistance to more critical levels. The first site shows this resistor as a switch pot, I would recommend trying this.
I am yet to finish making up a set of coils and a calibrated dial label for it. It is still an experimental device IMO, but it is quiet practical, I just use a counter with a coupling loop to work out where I am on the band.
Notes
The BF199 pin out leads to an elegant face-to-face gluing and soldering. This allows the collector-base connections to be extremely short and helps with the HF response and to prevent parasitic resonances. The emitters are also easy to connect and take to ground through the resistor.
My use of a piezo element and its connection across the collectors of the AF oscillator is problematic. The element in question has a very high mechanical Q and it tends to mode the oscillator near harmonics and sub harmonics of its natural frequency. This is an issue I've largely ignored, except for shunting the element with a resistor, which didn't help much. I tried collector to each rail, but that didn't help a lot either. I'm yet to try another element or a magnetic sounder.
The frequency of the AF oscillator is around 3.27 kHz and it shifts down typically to 3.16 kHz, this is audible, but could be better.
This project inspired a new nano-Henry meter adapter device.
A small buffer feeding a counter output would be a good addition.
Yep, I soldered straight to the AAA battery. Note also the elegant battery retaining device, a piece of the carrier strip for one of the 100 nF capacitors.
The Super-regenerative Tone Dipper is a much better device, and has replaced this unit.
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