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 Post subject: RF probe for fluke dmm
PostPosted: Jun Wed 05, 2019 9:48 pm 
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I have a fluke 115 meter, im looking for a schematic to build a rf probe to read RMS . What im doing is checking the crystal oscillators on my yaesu ft 101. The manual says if you have .03 volts on all the bands there in alignment. I just need to check it. I have a vtvm but i was wanting to use a dmm . I was wondering if i could check it with my oscillascope. I have adjustable 1-10x probes.
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 Post subject: Re: RF probe for fluke dmm
PostPosted: Jun Wed 05, 2019 10:59 pm 
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A passive RF probe using semiconductor diodes isn't going to let your DMM read down to 30 mV RMS. The Fluke 85RF probe is only good down to 250 mV:
http://assets.fluke.com/manuals/85rf____iseng0500.pdf

To use your scope, you'll need to use a 10X probe to keep from loading the circuit. Can your scope read down to 10 mV per division? Does it have sufficient bandwidth for the highest frequency you need to check?

Measuring small RF voltages is not easy.

Edit: I meant to write "1 mv per division".

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Last edited by stevebyan on Jun Thu 06, 2019 12:18 am, edited 1 time in total.

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 Post subject: Re: RF probe for fluke dmm
PostPosted: Jun Wed 05, 2019 11:25 pm 
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My scope goes down to 1mv per devision. 50mhz. . Sounds like i should be good if the 10x probe doesnt load it down too much. Is there any need to build a rf device to change it to dc ?

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 Post subject: Re: RF probe for fluke dmm
PostPosted: Jun Thu 06, 2019 12:19 am 
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devildog63 wrote:
Is there any need to build a rf device to change it to dc ?

Not if the frequencies you are measuring are well below 50 MHz. That 50 MHz is probably the -3 dB bandwidth, so a 30 mV 50 MHz sine wave would only read about 21 mV.

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 Post subject: Re: RF probe for fluke dmm
PostPosted: Jun Thu 06, 2019 1:05 am 
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Right, i believe 30mhz is the largest signal i might would measure. Maybs later i might measure something larger but not at this point.

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 Post subject: Re: RF probe for fluke dmm
PostPosted: Jun Thu 06, 2019 4:04 pm 
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Is that .03 volt a RMS value or peak voltage??? Your scope measures Peak to Peak voltage. RMS will be 2 X ( VP = VRMS × √2)

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 Post subject: Re: RF probe for fluke dmm
PostPosted: Jun Thu 06, 2019 8:29 pm 
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Yes peter it's a RMS value. So what formula do I need to use for vrms?
Thamks

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 Post subject: Re: RF probe for fluke dmm
PostPosted: Jun Fri 07, 2019 5:19 am 
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devildog63 wrote:
I have a fluke 115 meter, im looking for a schematic to build a rf probe to read RMS . What im doing is checking the crystal oscillators on my yaesu ft 101. The manual says if you have .03 volts on all the bands there in alignment. I just need to check it. I have a vtvm but i was wanting to use a dmm . I was wondering if i could check it with my oscillascope. I have adjustable 1-10x probes.
Thanks


Does the manual recommend an instrument to use ?

If not , can you post a partial schematic of where the pick off point is ?


What I'm curious about is the source impedance of the test point.

If its low enough, your X10 probe capacitance will not affect it.

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 Post subject: Re: RF probe for fluke dmm
PostPosted: Jun Fri 07, 2019 12:20 pm 
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If it is the USB LSB carrier oscillators, an important factor is the frequency. Have a good counter to adjust the trimmers. I believe there is a trimpot in those radios to make sure the transmitter and receiver are operating exactly on the same frequency. Otherwise I have heard some people tune in other stations and end up 100 hz or more off on transmit.

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 Post subject: Re: RF probe for fluke dmm
PostPosted: Jun Fri 07, 2019 1:00 pm 
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devildog63 wrote:
Yes peter it's a RMS value. So what formula do I need to use for vrms?
Thamks



Just use the online calculator if it is easier; make sure to use the pull down menu to select either Peak or Peak to Peak.

https://www.allaboutcircuits.com/tools/ ... alculator/

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 Post subject: Re: RF probe for fluke dmm
PostPosted: Jun Fri 07, 2019 2:55 pm 
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devildog63 wrote:
Yes peter it's a RMS value. So what formula do I need to use for vrms?

Vrms = Vpeak * 0.707, or
Vrms = (Vpeak-to-peak/2) * 0.707

These assume the waveform is a sine wave.

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 Post subject: Re: RF probe for fluke dmm
PostPosted: Jun Fri 07, 2019 8:40 pm 
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Thanks peter i seen the formula after i posted.

The manual states using a vtvm for rms with high impedence.

My frequency counter isnt accurate enough . I need to aquire a external 10mhz source.

The only reason im checking this is because the slug for T101 on the preselector has to be adjusted all the way down in the can to get tx and rx aligned on 40 meters. Thats not right. I havd aligned everything carefully several times according to the service manual.


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 Post subject: Re: RF probe for fluke dmm
PostPosted: Jun Fri 07, 2019 9:43 pm 
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So if you have a 'scope, why can't you just display the output as a sine wave and adjust the capacitor for max smoke then measure the peak-to-peak voltage and see that it's greater than 30 x 2 x 1.414 = 84mV. ?

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 Post subject: Re: RF probe for fluke dmm
PostPosted: Jun Fri 07, 2019 10:32 pm 
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Oscillator diagram is good, but where is the local oscillator test point ?

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 Post subject: Re: RF probe for fluke dmm
PostPosted: Jun Fri 07, 2019 11:36 pm 
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Max smoke? Lol. Thats what im asking, why cant l ?
Heres a picture on the TP next to 15. Let me know you guys thoughts.


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 Post subject: Re: RF probe for fluke dmm
PostPosted: Jun Sat 08, 2019 2:56 am 
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Where does the wire from the drain of Q15 go ?
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 Post subject: Re: RF probe for fluke dmm
PostPosted: Jun Sat 08, 2019 3:07 am 
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It gos to L11 on the mixer board. Hers the schematic .


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 Post subject: Re: RF probe for fluke dmm
PostPosted: Jun Sat 08, 2019 3:07 am 
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Correction , its L1

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 Post subject: Re: RF probe for fluke dmm
PostPosted: Jun Sat 08, 2019 3:42 am 
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Block diagram.


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 Post subject: Re: RF probe for fluke dmm
PostPosted: Jun Sat 08, 2019 1:01 pm 
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If my memory is correct, wasn't one of the preselector coils already changed due to a cracked slug in the tuning rack assembly??

LO injection should NOT affect your RF preselector tracking. It has been a while since I looked at one, but are there fixed or trimmer caps associated with aligning the tracking for each band range? It almost sounds like might have an open trimmer cap if only one band is not tracking.

Pete (W1BR on the "zed")

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