30-11-2023, 06:14 AM
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
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
Doug VK4ADC @ QG62LG51
http://www.vk4adc.com
This Forum is only going to be as interesting as the posts it contains.
If you have a comment or question, post it as it may trigger or answer the query in someone else's mind.
http://www.vk4adc.com
This Forum is only going to be as interesting as the posts it contains.
If you have a comment or question, post it as it may trigger or answer the query in someone else's mind.