QST article on using low bandwidth scopes for HF work

[Opcom]

In the last 1-3 years, there was a QST article on an adapter to go between a scope and the transmitter that would allow an old low bandwidth scope to display an AM envelope coming from a transmitter at a much higher frequency than the scope could handle. The circuit was pretty simple, an IC or two and some other stuff. I recall the mention that it was a similar technique as used in digitizing scopes, but the article required no digital memories or complicated stuff, and was to be used with analog scopes.

Does anyone recall when this was published?

Update; by luck, it turned up, the QST's were misplaced!! January 2013. Sorry for the trouble.

[WD8KDG]

One method was to bypass the vertical amp section of the o'scope. Using a sampler/pickup to fetch some RF to view, send it straight to the vertical deflection plates of the CRT.

Long ago I used a project box to house several toroidal cores & an air variable cap. Each core wound with some magnet wire and would be resonate in a band by adjusting the air variable cap. Also placed in the project box was the voltage divider circuit used to view a trapezoidal pattern.

All this JS stuff is still doing the job for an old Eico 460 o'scope on the Junkston 500 station.

Works gud, last long time.

Craig,

[KD6VXI]

There is an adapter based on  mini-circuits module in most arrl handbooks.   I k of its in the 1991, because I have that one.   It's in the garage packed,  or I'd snap a pic of it.

Very simple,  LO coupled to a mixer.

--Shane
KD6VXI

[Steve - K4HX]

Given that you can get a 20-30 MHz scope and most any fest for less than $50, such a circuit hardly seems worth the bother. If you want to have some vintage scope to make vintage gear, then OK.

[Opcom]

The deflection plate way is the best for simplicity when you have 30V or more (CRTs offer 20-100 volts per inch deflection depending on type and accelerating voltage) , but if you have only a few volts such as from a modulated oscillator, then this sampling way seems good. It is not a downconverter but a sampler. There is not so much need to change LO frequencies as with a down converter so that is an improvement.

It uses a very narrow pulse to gate an amplifier that passes the RF signal to be measured. The pulse repetition frequency is set close to a sub harmonic of the desired frequency.

To take from the article, if the gate pulse is repeated at 1.75 Mhz, then the 8th harmonic is 14Mhz. Therefore a 14.15Mhz signal would be 'converted' to 150KHz, and a 14.35MHz signal would be converted to 350Khz.

The input signals are sampled at a rate of 1.75Mhz, but the actual samples, that is the instantaneous voltage levels of the 14 Mhz signal at the precise instant of the pulse, are what come out of the gated amplifier.

The result is a train of voltage samples following an amplitude curve or pattern that is equal in frequency to the beat between the 8th harmonic of 1.75Mhz and 14.15MHz.

The 8th harmonic of the 1.75MHz pulse train does not exist as a signal really, and one can say only 1 of 8 pulses is applied because the sample rate is 1.75MHz and not 14Mhz, but it does not matter. The equivalent statement is that 1 in 8 possible samples are captured.

It is enough, because a time constant after the amplifier smoothes out the voltage. Like a detector.

So, this I.F. signal is sent through a 500KHz LPF, and results in a 0Hz to 500KHz band width (limited by components to a lower limit of 100Khz).

Thus a 14.15MHz signal is making a 150KHz signal and a 14.35MHz signal is making a 350KHz signal. A 14Mhz signal would give a 0Hz output.. so it has certain points where it may not be as useful. It is suggested that the '1.75Mhz' crystal could be made a variable or switchable frequency to accommodate other measurements. The requirement in the given circuit is that the sampling frequency (e.g. 1.75MHz) represent an Nth harmonic frequency within 100-500Khz of the signal to be measured. A lower limit of 100KHz is not really enforced except by the fact that the output signal, if derived from a modulated RF envelope, would become a modulated envelope of such a low 'RF' frequency as to not be 'filled' by the many many cycles of the audio modulating signal that is expected.

These low frequency signals are easily displayed on almost any old scope, even the hallowed RM-503. Because this is so wide a bandwidth, it is best used for a transmitter or signal generator or what-have-you that makes just the one signal which is of interest. The article says that the RF conversion response is rather flat and can be used up to 6 meters.

There is a picture of the input, pulse train, and output waveforms in the article. It is a bit misleading because the width of the pulses would be narrower, and a 8th harmonic example is given in text but the figure shows about a 2nd harmonic example. However it is explained in the caption of the figure. A scope going up all the way to 50 or 100MHz is still somewhat expensive, so it is a worthwhile experiment.

[WD8KDG]

Given that you can get a 20-30 MHz scope and most any fest for less than $50, such a circuit hardly seems worth the bother. If you want to have some vintage scope to make vintage gear, then OK.

Steve........yup! But way back then in the early 80's, with wife & kids, a cheap Eico 460 was stretch'en the budget. A few more bucks and I cobbled together something that is still working. Something to be said about passive components & tubes.

Craig,

[Steve - K4HX]

Neat!

[Opcom]

JSo'scope.pdf - that is a well thought out circuit. I been collecting up the old $5 scopes.. HV works = potential mod scope.