Easy BitX build - RF probe

[VK3UVW]

Hi there,

I'm building a kit, the 'easy bitx' from inkits. Been a bit of a slog, but almost there! 

At one point it requires an RF probe be used to test signal on the pcb. I did find a howto but the ascii schematic is a bit unclear.

A) Am I right in thinking there's a capacitor (C1) at the tip, a diode(D1) to ground, then a 1M resistor (R1), then another capacitor(C2) to ground, with measurement taken across C2?

B) Is measurement just with a standard digital multimeter ok? Or is a moving coil type the only real solution?

C) Is there any advantage to this method over just probing with an oscilloscope or spectrum analyser (I have both)

D) Is impedance of the RF probe a concern at all?

E) Does the voltage output depend on the rf probe construction / component selection?

F) Lastly, does the paragraph

"While doing this test you would see that the meter needle if analogue meter is used and some voltage show’s up to a certain level, this is the carrier from the balance modulator which has to be made zero or null. To null the carrier press PTT with RF probe connected adjust RV2 100 ohms in balance modulator till the meter needle comes down to zero, adjust the 22pf trimmer too. So when you modulate now you want see any movement in meter apart from a ticking of 67 meter, so when you modulate now the meter will move and come back to zero when no modulation is done" 

make much sense? (Taken from page 66 of the build instructions). I know where RV2 is on the board; not entirely sure how the balance modulator works but my vague understanding is that it suppresses the carrier so only the LSB/USB are present (Thanks, google). I guess my main question is what the 67 meter is; and is "modulation" in this sense just transmitting?

I'm building this thing because I want to learn, and it's been a success on that front, but now I'm at the pointy end of making it actually work...

Thanks for reading,
Scott

[VK4ADC]

Scott

I have edited the RF probe image and denoted the actual parts soldered in place to make it easier for you to re-create, similtaneously visualising the schematic.

   

To answer your various questions..

1.  there is a small SMD capacitor in series to the tip, and another across the wires at the cathode (K) of the diode across to the end of the 1M resistor.

2.  you queried whether the K of the diode was connected to ground, and the 'normal' answer is no if you want a high impedance probe. Yes you could ground one end BUT then the impedance varies between the conducting resistance of the diode and O/C every RF cycle, resulting in a much lower RF probe impedance.  Connecting the diode in the series-leg to the output (rather than ground) makes for a higher impedance and lower loading on the circuit-under-test.

3. advantages - the RF probe will provide an absolute value of voltage which makes repetitive use easier, and is subsequently easier to peak or dip for maximum/minimum indication, while the SpecAn or will require a lot more adjustment for a simple procedure and requires a lot more interpretation to make the same circuit adjustment.  The typical oscilloscope works fine at low frequencies but seldom at higher RF frequencies.

4. the probe's output voltage level is determined only by two things : the amount of RF being evaluated; and the loading on the probe by the DMM/multimeter's input resistance (i.e. the circuit loading). The DMM usually shows the highest probe output voltage due to its high input resistance over the multimeter BUT the pointer movement on an analogue meter makes it easier to make peaking or dipping adjustments.  Variations in the actual component values will make little change to the output voltage provided the value ranges indicated in the article are followed - noting that the final values are not critical.

5. "I know where RV2 is on the board; not entirely sure how the balance modulator works but my vague understanding is that it suppresses the carrier so only the LSB/USB are present" - Yes, the balanced modulator "balances' out the carrier signal leaving only both sideband components (ie USB and LSB) . It is the function of a sharp filter following the balanced modulator that is used to take out one sideband - or the other.  The 'level of balance' is what RV2 is there for and why you would set its position for a minimum RF probe voltage.

It may be worthwhile trying to get your hands on the likes of an old ARRL Handbook - any year from about 1980 on - and do some reading of various chapters to make that learning process easier - without needing Google - all of this stuff is described in detail in that book.  You could possibly even find a local amateur who is willing to donate an old copy to a good cause, and maybe more importantly have time to assist you in your learning process..

Doug VK4ADC

[VK5PJ]

Hello Scott,
further to Doug's reply, the beauty of using a crude RF probe is that it is DC isolated by design and a relatively high impedance device, reducing the loading on the circuit under test. You only need a relative reading and not anything fancy to do the task.

In the good old days, the RF probe would be connected to the shack multi meter with a moving coil meter which are fantastic for Peak and Dip jobs.

Do not over think it, the task is to just NULL the carrier level in the balanced modulator as best you can. Older SSB transmitters all had what is called carrier leakage and it was and acceptable part of the early days of SSB

Hope it turns out to your satisfaction.

[VK3UVW]

Thanks Doug and Peter, that's very helpful - particularly the marked up diagram.

It may be worthwhile trying to get your hands on the likes of an old ARRL Handbook

I do have the 6 volume set, am almost finished the 2nd book. Rate I'm going I should be done with the set early 2025. I didn't find anything that seemed to match the RF probe, tho I must admit I didn't look up the balance modulator.

Do not over think it

One day I'll learn this trick. Today is not that day

[VK5PJ]

Hi Scott,
your RF probe does not need to be exactly what's in their documentation, here is a link to a USA ham who has a page describing a simple RF probe: https://n5ese.com/rfprobe1.htm and how you can build it. He also goes into some of the background and uses of an RF probe.

I must admit I do not have one around my shack in any shape or form so this might inspire me to build a simple probe into a case and hang it with the other test leads I have.

And yes I will say it again, do not over think it, too many projects get put to one side because we over think things, me included