EMRFD Message Archive 15053

Message Date From Subject
15053 2018-09-08 09:21:05 peter_dl8ov Two Tone RF Amplifier Measurements
If measuring the IMD of an RF amplifier how far apart should the two signals be? My manager reckons that there should be less than 3 KHz between them as this will be the bandwidth of an SSB signal, but that doesn't sound right, and stopping the two crystal oscillators from locking to each other would be a nightmare.

I was thinking about using 14.1 MHz and 14.2 MHz. Any comments or suggestions?

Peter DL8OV


15054 2018-09-08 09:43:55 Russell Shaw Re: Two Tone RF Amplifier Measurements
15055 2018-09-08 10:51:26 Jochen Feldhaar Re: Two Tone RF Amplifier Measurements

Hello Peter,

the IM3 measurement is a function of the distance of the 2 independent carriers. If too near to each other, LF behaviour of the biasing circuitry will have amajor influence in the result, if too far apart, they will be distorted by the limited bandwidth, or their respective amplitude will decrease, thus invalidating the result. So first you should determine the useful bandwidth of the amplifierunder test, and then choose the 2 frequencies at a third to half the calculated bandwidth.

The result will be a function dependent on the frequency difference.

If you have further questions, please feel free to ask me for more information.

Regards, Jochen DH6FAZ

m 08.09.2018 um 16:25 schrieb peter_dl8ov@yahoo.de [emrfd]:

 

If measuring the IMD of an RF amplifier how far apart should the two signals be? My manager reckons that there should be less than 3 KHz between them as this will be the bandwidth of an SSB signal, but that doesn't sound right, and stopping the two crystal oscillators from locking to each other would be a nightmare.

I was thinking about using 14.1 MHz and 14.2 MHz. Any comments or suggestions?

Peter DL8OV


15061 2018-09-08 18:54:35 Ashhar Farhan Re: Two Tone RF Amplifier Measurements
The IMD measurements wil be the same as long as you do not have any filtering in the device under test between the two intermixing frequencies.
It is a different scenario for a complete radio. Let's imagine a regular superhet like the S7C of the EMRFD or the bitx. They have a bandpass filter at the RF band, followed by a crystal filter at the IF. In such a narrow band design, if you were measuring IMD to evaluate spurious immunity, you would do it with a spacing of two signals such that each is sufficiently away from the other to be out of the crystal filter response. On the other hand, if you were measuring the in-channel distortions, you would need to have the spacing such that both the signals are within the table top of the crystal filter shape. 
- f

15066 2018-09-10 11:35:24 kb1gmx Re: Two Tone RF Amplifier Measurements
For amps in general two non-harmonically related tones.

If your testing a SSB radio that usually means one below 1500hz like 700hz and 
one above  such as 1750hz.  You want at both to fit inside the TX band width which 
maybe less that 3000hz and be as narrow as 2100hz.

For a tuned amp the tones again non-harmonic and withing the band pass.

For wideband they can be as far apart but again non-harmonic.

Keep in mind are you testing the amp or the radio (or test gear) driving it
as both may more may not be better than the amp.

Allison