VHF wide band oscillator design - Page 1
Forum section: electronics / rf-microwave

author: szoftveres
author_rank: Regular Contributor
timestamp: August 17, 2019, 10:21:49 pm
content: Hello Community,I'm designing a receiver that would cover the VHF band from 4m to 2m (88 MHz to 146 MHz); my idea was to create a universal front-end that would cover the entire range.For the local oscillator, I chose the Hartley topology in the hope of a somewhat constant amplitude, and built the prototype on a copper-clad board ("Manhattan-style"):During testing, I observed a somewhat weird behaviour to which I couldn't get an explanation for:When the oscillator is biased for higher idle current (low R1), the oscillator is tunable through the entire frequency range, as expected (the amplitude is not as constant as I expected, though). However, when biased for low idle current (high R1), the frequency "jumps" as I tune up- or down: e.g. the frequency goes from 88 to 100 MHz, then suddenly jumps to 110 MHz and continues to go up. The frequency "profile" even has hysteresis, e.g. when I tune from high- to low, I can tune below 110 MHz before it jumps down to ~98 MHz. etc..I built the oscillator with a bipolar transistor as well (MPSH10, with the necessary biasing added) and observed a very similar behaviour.Can someone shed some light on why this happens? Also, is there a good way to stabilize the amplitude of the oscillator so that it would remain more constant throughout the entire frequency range?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 18, 2019, 02:21:48 am
content: Is there really no gate bias on the top transistor?Does the inductor have any odd modes to it, or is it practically ideal over the operating range?Tim

author: szoftveres
author_rank: Regular Contributor
timestamp: August 18, 2019, 04:11:52 pm
content: There's a gate bias resistor in parallel to the 10p capacitor, apparently I missed it on the drawing.Also didn't note on the drawing (because I thought it was irrelevant) the fact that C1 is two BB139 varicaps, facing each other, the tuning voltage is fed into them by a 100k resistor.Just like on this drawing:I replaced the varicaps with a real trimmer cap, and the issue disappeared; I was able to bias the oscillator to a very low idle current, without the oscillator showing any band skipping.Also, I didn't see the issue after having added a sufficiently large (e.g. 10p) capacitor in parallel to the varicaps (of course at the cost of a significant band spread reduction).So I suspect that the issue is maybe the intolerance of the varicaps of the oscillation amplitude. Ultimately I'd like to tune the circuit with varicaps, so I'm still curious about the background of this issue and how other, varicap-tuned oscillators solve it.The inductor is a Coilcraft 144-04J12L, I added two taps to it. S.R.F. is around 700 MHz.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 18, 2019, 04:56:40 pm
content: Heh, I wonder if there's unbalanced capacitance on the tuning node, and that's causing rectification and pulling.Can also try an inductor instead of resistor for bias.General advice would be to keep the signal strength down, so the varactors look more linear; you also get less distortion for the same reason. Can always amp it up later (at the expense of some SNR loss), bonus is the amp gets you isolation so it's less sensitive to load pulling.Tim