RF Level Characterization

While trying to measure the frequency response levels of a mixer, I was having problems accounting for the observed mixer gain.  I do not have a high quality wide ranging frequency source.  I use synthesizers built from previous efforts.  These work fine with respect to frequency accuracy and phase noise.  I knew they were not designed as signal generators and had some level variance.  For my most purposes this was fine. With this activity I needed a better estimate.  Below is the output power level at the fundamental of a ADF4351 based synthesizer.

This was taken using a SA0314 using the max hold functionality while scanning 1GHz at a time.  Beyond the expected roll off with frequency (largely due to PCB layout and board) there is a ~90MHz swing of +/- 2dB.  There is a slightly larger variance at select frequencies.  My first thought was to calibrate it out and use the 3dB power output steps of the part to get something a little better.   Unfortunately the variance changes with load and between parts.  I knew this from reading the support forums but also observed it by comparing the maximum at different frequencies through different mixers, amplifiers and load combinations. So if you are using these items in their intended application (i.e. a LO with only a minimum output requirement) its fine.  If, on the other hand, you are using it as the RF source and sweeping them across a fixed LO you end up measuring the source variance (+/- 2dB), not the mixer gain.

Beagle Bone Black Mixer Board (Prj133)

The B board took some short cuts with the mixer input and output impedance matching for a variety of reasons.  I wanted to revisit the mixer used and focus on level control, higher input levels, and a broader band match.  The board shown below uses a LTC5510 broadband mixer.

The prototype (Prj133) includes a layout for an on-board ADF4351 synthesizer.  It uses a broad band 1:1 input balun and has provisions for a SMT high frequency output balun or a discrete lattice balun.  The board gets its power and digital interface from a Beagle Bone Black interface board (I board).  The digital controls are only used for the synthesizer.  Provisions were made to allow an external LO input in cases where the synthesizer is not populated. (NOTE: the mixer is a 0.65mm pitch part and not 0.5mm, and no when you make this mistake you can't get it to work with careful soldering/seating)

Mixer Board Using LTC5510

The following is a picture of the first unit built (synthesizer unpopulated to focus on mixer measurements).

Synthesizer (ADF4351) and Mixer (LTC5510) Board.  Power and digital interface at left via 2x5 header.  Synthesizer not populate.  External LO input from bottom center SMA.

The output sections does show a fair amount of rework.  I actually started with the high frequency SMT output balun.  I had enough problems mounting this part that I decided to back up and start with something simpler.

IF output on left, RF input on right.  Mixer in center. Discrete 4:1 Balun at left of mixer.

This output balun was designed as a 4:1 impedance match around 10.7MHz.  It was constructed using the component values shown in the following table.

4:1 10MHz Balun Part	Value	Comments
C105/C113	10nF	DC blocking capacitors
L102/L103	10uH	DC supply chokes
L107/L108	1500nH	4x Lattice Balun inductor
C106/C107	150pF	4x Lattice Balun capacitor

 The following graph captures the difference product across a swept LO using a 50 ohm LO (~ 0dBm) and RF (-15dBm) source.

Mixer IF output using a 4:1 lattice lalun

The output level of -13.2 dBm with an input level of -15.0 dBm is within expectations.  It is +1.8dB.  The datasheet indicates a 3.3V supply at 1575MHz IF output of +1.5dB.  We expected to loose a fraction of a dB on the input balun in the 1200MHz range and a fraction of a dB on the output balun due to losses as well as 245 ohm vs 200 ohm mismatch.  At this IF we would expect a slightly larger mixer gain (the datasheet does show a 44MHz IF output on page 13 but the gain is still showing 1.5 – 2 dB).  Given the uncertainty in the measurement values (at least 0.5dBm)  the device and balun are acting as expected. The output level was also verified to be constant with a varying LO from -12dBm to 0 dBm (as per datasheet results).