« on: December 30, 2019, 02:17:07 pm »
I'm trying to trace the cause of the RF Oscillator instability in my Philips PM5324 RF signal generator. The instability consists of distortion (sine wave goes out of shape the shape of the distortion changes with frequency) and the amplitude reduces gradually until around 32 MHz (most instability centres on 31.8 MHz or thereabouts) and then the amplitude rises again. The distortion is still present right up to around 110MHz. Up to 10MHz the sine wave signal is stable in amplitude and shape.
Some questions that someone may be able to help me with.
1/ In the snapshot of the schematic below showing the HF Oscillator section what is the topology of the oscillator (i.e Wien Bridge, Colpitts etc)?
2/ What influence would noise on the power supply have on the stability of the Oscillator. In particular could failing electrolytic capacitors introduce HF noise.
3/ The power supply regulator is 741 op amp and Darlington transistor based. Could a fault here be a cause?
I'm going to do some voltage checks tomorrow, so I'll report back. I thought at first that the switches are the likely cause, but they seem OK. Then I had a look at the output amp (no voltage checks yet though). I found that the distortion is present at test point 10 in the schematic so it originates at the Oscillator itself.
More checks in the morning. Just wondering if anyone has any additional things I should be looking for? Thanks.
P.S Reading through the description of how the Oscillator works (attached) I have noticed a 47uF capacitor (Ref 530). It seems that this is involved in the amplitude stabilisation. I wonder if this is the faulty component? I shall investigate in the morning.
« Last Edit: December 30, 2019, 03:16:37 pm by djsb »
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David
Hertfordshire, UK
University Electronics Technician, London, PIC16/18, CCS PCM C, Arduino UNO, NANO,ESP32, KiCad V8+, Altium Designer 21.4.1, Alibre Design Expert 28 & FreeCAD beginner. LPKF S103,S62 PCB router Operator, Electronics instructor. Credited KiCad French to English translator