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Old 2nd Oct 2020, 10:45 am   #15
Join Date: Jun 2016
Location: Bognor Regis, West Sussex, UK.
Posts: 2,272
Default Re: Adding a Timebase Output to an old Scope

Thanks for the update David.
Actually if R5 is made 10 ohm the circuit will work with either a normal 555 OR the Cmos version.

The circuit is very good. Q1 is a constant current generator that uses 4 series diodes as a voltage reference and R9/R2 set the current.
The constant current charges C1 in a linear fashion. U1a buffers the ramp and provides the output.
The ouput also goes to U1b, a comparator, and when the ramp reaches a threshold set by R3 & R6 the output of U1b goes low.
This triggers the 555 which is configured as a one shot monostable multivibrator.
The output of the 555 on pin 3 goes high for the period determined by R4 & C3, 0.24mS.
This turns on Q2 which discharges C1. When the monostable multibrator times out the output goes low again, Q2 turns off and the cycle repeats.

While the Cmos 555 works with R5 as 10K it will affect the timing of the monostable, I would recommend making R5 10 ohm in all cases.

I am looking to build the Haigh wobbulator hence my interest. I have managed to get equivalent coils and Varicap diode.

The reason I am going to use a current mirror for the current source is temperature stability. The Armstrong design uses 4 diodes as a voltage reference, one will be compensated by the base - emmitter junction of Q1 but that leaves the temperature coefficient of 3 diodes in series, 6mV/degC.
The current mirror is more stable with temperature change provided the two transistors are closely coupled thermally.

The Haigh wobbulator article is also in ETI Vol 27 #13 available from Worldradiohistory


Last edited by Electronpusher0; 2nd Oct 2020 at 10:49 am. Reason: Link to the wobbulator article
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