25-11-2013, 09:34 AM
I've re-read the article to ensure that I understand how (in theory) the RF probe is supposed to operate. Its aim is to take a low level RF signal and to amplify it to enable the response curve to be displayed on the 'scope screen. At the bottom of Page 27 of the article it shows two traces of what to expect at the output of the probe. I've done a few quick tests and have injected a 1 MHz signal and have shown the traces obtained below. In all cases the scope is set to 0.5V per division.
The first pic is at the input to the probe tip, ( about 0.75V P to p); the second is at the output (cathode) of D1 with the probe set to the 'low' setting; the third pic is with the probe in the high setting (about 3V p to p), which is about which indicates that the J310 amplifier stage is working. The fourth pic shows the scope trace with no connection to the probe to show the baseline I set the scope to. The final pic is at the output of the probe with the scope set to DC, which shows the trace lifted by about 3V from pic 1. (With the scope set to AC, there is no AC output - ie RF waveform - from the probe). This seems to indicate that D2 - rather than working with D1 as a voltage doubler to display low level RF signal inputs to the probe - is simply acting as a rectifier, rectifying the 1 Mz input to D2 to DC at its output. Thus, the probe will give a DC voltage output of an RF signal input, (as stated in the top para on the second column of page 27 of the article) but will not display an RF waveform as indicated by the two pics at the bottom of page 27.
I remain perplexed - I'm having a Donald Rumsfeld moment - it's a 'known unknown' !
Small project - I might just abandon it and build another, but it's often said that to do the same thing over again and to expect a different outcome is the first sign of insanity.
Confused, of Cottingham.
The first pic is at the input to the probe tip, ( about 0.75V P to p); the second is at the output (cathode) of D1 with the probe set to the 'low' setting; the third pic is with the probe in the high setting (about 3V p to p), which is about which indicates that the J310 amplifier stage is working. The fourth pic shows the scope trace with no connection to the probe to show the baseline I set the scope to. The final pic is at the output of the probe with the scope set to DC, which shows the trace lifted by about 3V from pic 1. (With the scope set to AC, there is no AC output - ie RF waveform - from the probe). This seems to indicate that D2 - rather than working with D1 as a voltage doubler to display low level RF signal inputs to the probe - is simply acting as a rectifier, rectifying the 1 Mz input to D2 to DC at its output. Thus, the probe will give a DC voltage output of an RF signal input, (as stated in the top para on the second column of page 27 of the article) but will not display an RF waveform as indicated by the two pics at the bottom of page 27.
I remain perplexed - I'm having a Donald Rumsfeld moment - it's a 'known unknown' !
Small project - I might just abandon it and build another, but it's often said that to do the same thing over again and to expect a different outcome is the first sign of insanity.
Confused, of Cottingham.
Regards, David.
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
