How this oscillator works!? - Page 1
Forum section: electronics / rf-microwave

author: Gregory
author_rank: Regular Contributor
timestamp: June 25, 2025, 01:10:20 am
content: Hey guys,This is the oscillator of the HP5350B frequency counter.How do you think it works? It is certainly not a "common" topology.May it be a reflection oscillator using Q1?

author: D2236
author_rank: Regular Contributor
timestamp: June 25, 2025, 04:03:20 am
content: It appears to be a Clapp oscillator(with crystal based inductance), using a Colpitts-style feedback network.

author: rf-fil
author_rank: Regular Contributor
timestamp: June 25, 2025, 04:32:18 am
content: Common collector stage with C8, C5/C6 forming a Colpitts(-ish) oscillator, with crystal in inductor position. The crystal is grounded via C1, which would make it a Clapp topology. C1 and related parts are grounded via the low input impedance of the Q2 common-base stage. The C5/C6/L2/L3 network is interesting and I would speculate that they are traps to prevent overtone oscillation, while the C5/C6 capacitors are large enough to provide the Colpitts oscillator feedback. C8 is connected between collector and emitter, but the collector is grounded at RF via C7 - it becomes easier to see this as a Clapp if re-drawn with C8 going to ground.

author: D2236
author_rank: Regular Contributor
timestamp: June 25, 2025, 04:13:08 pm
content: I simulated the circuit, taking some calculated guesses on the internal crystal values. In any case, the results seems to show very good harmonic suppression, and a healthy > 10 dBm output.

author: Gregory
author_rank: Regular Contributor
timestamp: June 26, 2025, 02:08:24 am
content: Realizing C1 to RF ground explains a lot.Fascinating use of L3, L2, C2.A parallel resonance (A) below 10 MHz, (L3 + L2) + C2, allows the negative impedance to surge (allows Vbe excitation).A series resonance (B) above 10 MHz, L2 + C2, kills the negative impedance (kills Vbe excitation).The negative impedance transitions crossing 0 phase (~0 imaginary) right at 10 MHz.Broadband negative resistance without L3, L2, C2.

author: KE5FX
author_rank: Super Contributor
timestamp: June 26, 2025, 02:42:10 am
content: The HP 10811 is pretty well-documented. One of the all-time greats from the OCXO aficionado's point of view.Some of the best oscillator gurus at HP spent a fortune trying to do better, and never really succeeded. Lots of leading-edge papers and patents, but at the end of the day, the best 10811s are still just as good or better than any later-generation oscillator the company produced.

author: iMo
author_rank: Super Contributor
timestamp: June 26, 2025, 05:25:30 am
content: Quote from: rf-fil on June 25, 2025, 04:32:18 amFrom the above Manual - it is a 3.3333MHz crystal, it seems (most stable cuts are till 5MHz, afaik)..Quote

author: tchicago
author_rank: Regular Contributor
timestamp: June 26, 2025, 07:34:38 am
content: An interesting CR2 element. Apparently it is a stable-current diode - 1N5297 according to the manual. Never heard of such before.$16 on Digikey, so I guess any kind of current-regulating contraption from the college textbook would be cheaper than thisExcept if price is not a constraint, like in this high spec OCXO.

author: iMo
author_rank: Super Contributor
timestamp: June 26, 2025, 11:14:42 am
content: It could be a current source (ie from Siliconix), a jfet+resistor inside, they offered those for a range of currents (1.6-4.7mA)..

author: iMo
author_rank: Super Contributor
timestamp: July 03, 2025, 12:03:19 pm
content: The OP is wrapping up..