HG-10 VFO out issue's

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AB2EZ:
I descided to hook my HG-10 (working, but rarely used) to my SB-10 sideband adapter. As expected, the signal that the HG-10 developed across the input of the SB-10 was too low, particularly on 75m

Note that the SB-10 has an input circuit that looks roughly like a 1000pF capacitor (around -j40 ohms on 75 meters) in series with two parallel +/- 45 degree phase shifting networks. Each of the phase shifting networks has an impedance of (roughly) 50 ohms - j50 ohms on the selected band of operation.

Note that the HG-10 has a cathiode follower output circuit (could drive a low impedance)... but there is a 100pF capacitor (about -j400 ohms on 75 meters) between the cathode and the HG-10 output jack.

This 100pF capacitor in the HG-10 output path, in combination with the input impedance of the SB-10, acts like an approximately 10:1 voltage divider at 3.885 MHz.

Since the HG-10 was intended to produce a lot of harmonic content in its output waveform, one can understand why the the plate load on the emitter coupled oscillator is an inductor (higher impedance at higher frequencies)... and one can understand why there is a 100pF capacitor between the output of the cathode follower and the output jack (lower impedance at higher frequencies).

I am going to increase the value of the 100pF capacitor in the HG-10 output path to 1000pF... in order to substantially reduce the voltage divider effect on 3.885 MHz. I'll also check the inductor value to see what it is. I may increase the inductor value, and place a 10kohm resistor in parallel with it, to produce a load on the plate of the emitter coupled oscillator that is around 10kohms at all frequencies... since I don't use the HG-10 at frequencies above 7.3MHz, and I don't need a lot of harmonic content in the waveform.

Stu

k4kyv:
I seem to recall from somewhere that the 80m oscillator is actually oscillating on 160m, just as in the  case of the older VF-1, so that the 80m output is the 2nd harmonic.  Not sure if this is true, but might be worth checking out, and if so, it shouldn't be too difficult to modify the unit for output on 160m.

If you are using it to drive one or more class-C stages, harmonics in the output is of no consequence, since the class-C amplifier is inherently rich in harmonics in any case, depending entirely on the output tuned circuit(s) to filter them out.

Don't you love how "new and improved" often means useful features discontinued, such as Heathkit deleting the 160m band when they "upgraded" from the VF-1 to the HG-10, and from the DX-100 to the Apache?

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