Image rejection problem that I can't figure out

General discussion and support for the Peaberry SDR V2.

Image rejection problem that I can't figure out

Postby kb1mvx » Sun Nov 20, 2016 9:54 pm

I've had a Peaberry V2 kit on my shelf for two years and I am finally building it. I have it assembled through the the receiver section. The receiver works and tunes in HDSDR. I've tested it on 20, 30, 40, and 60 using a signal generator. The problem is the the image is as big as the the fundamental and the receiver sensitivity seems low. I checked all the solders, tried direct injection on C27, and rewound T2. No change.

Using the recording function in HDSDR I recorded some data with a tone 1 KHz away from the LO frequency. I read the data into Matlab for analysis. The amplitudes of the both I and Q are low but they are very close. The phase difference between I and Q is 50 degrees. It should be 90 degrees. Any idea where I am getting a 40 degree phase shift?

Jim - KB1MVX
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Re: Image rejection problem that I can't figure out

Postby kb1mvx » Mon Dec 05, 2016 3:00 am

I did more testing. I collected IQ data with the following LO settings:

5.060 MHz
6.060 MHz
7.060 MHz
8.060 MHz
9.060 MHz
10.060 MHz
11.060 MHz
12.060 MHz
13.060 MHz
14.060 MHz
15.060 MHz

In each case in injected a tone 1 kHz above the LO. The IQ phase difference ranged from 51 to 54 degrees across all of these frequencies so the phase shift is not happening at RF. It's happening at baseband. So, I don't have the answer yet but I'm getting there.

-Jim
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Re: Image rejection problem that I can't figure out

Postby kb1mvx » Wed Dec 07, 2016 3:25 pm

I'm going to switch gears and try to address the low sensitivity issue. In order to get ADC counts in the 400 range I have to push the RF drive level up to -10 dBm. That's obviously not right. I'm going to borrow an o-scope and work through the receive path.
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Posts: 3
Joined: Tue Jan 07, 2014 3:27 pm


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