So I just took my exam in RF Devices and Circuits and as I sat down at my desk I realized I made a big mistake. On my note card, I had an equation as instead of and that makes all the difference in the world. My instinct told me the answer I got was wrong but I double checked the math, using the wrong formula, and everything checked out. I wrote down all the work so I hope he sees the error and doesn’t hit me to bad for it.
I haven’t posted much after Monday because I wanted to avoid posts consisting of only four letter words. It was a bad week. I’ll be posting a summary of the week later on today.
Update: 11:00 PM
I started Tuesday with the realization that I made a big mistake on the mainboard. My receiver’s connector had its odd and even pin rows swapped. I could swear I’d checked that but apparently not well enough. Now if the receiver cards were lined up on the bottom of the board they would be correct, so I hastily decided to unsolder the connectors to reverse them. Until I realized that that won’t work because the LO connectors would now be offset in the wrong direction. However by this time, some of the traces were damaged in the unsoldering process because all the copper in the board quickly sucks away heat, solidifying unexpectedly. Now I didn’t trust the board. So rather than try and patch up that board and worry about any unseen damages, I decided to populate a new board to start with a clean slate.
With my new board and the connectors on the original side, I set about making some connectors that will dispace the pins 0.1 inchs. Luckly, I don’t really need both rows of pins as the even row is all ground connections and the LO connectors will provide sufficent ground for those low frequency signals. I was able to canbalize long pins from some scrap connectors and bend them to get a working connector though it was tedious work.
Now with working receiver cards, the next task was getting the ADC’s working. They were outputing all zeros but the problem was easy to fix. One of the pins on the new board’s ADC wasn’t quite soldered down well so I reflowed all the pins and I started to get data. Except that it was all wrong. But it looked familiar, as if the bipolar pin was set low to unipolar mode. Which it was. Worse yet, it’s a leadless package so I’m going to have to unsolder the part to cut the trace underneath and then wire it to 3.3V with a very small jumper.
For the record, here’s a list of known problems with the current version of the mainboard:
- The JTAG RESET signal is wired to the wrong pin.
- The transmitter 870MHz LO connector is offset in the wrong direction.
- The receiver’s connector has its even and odd rows swapped.
- The ADC’s bipolar/unipolar pin is wired to low instead of high.
It isn’t all problems though.
The MAX232 is very quiet with the new input power noise filter. The MAX232′s charge pumps were blasting out noise on the power line in a previous design and I had to enable/disable the drivers on every transmission. Now they’re not even detectable. I’ll be doing all MAX232′s with that filter from now on.
The 3.3V analog power line overall is very quiet; less than 5mV of noise on the scope which is pretty much its noise floor anyway. I took a preliminary look at it with the spectrum analyzer and didn’t see anything above -55dBm which was the nearby FM station at 94.7MHz. A shield room would be nice . The voltage regulators do get very hot though as the board pulls around 900mA without the transmitter. I need to add a heat sink across them. Something like this perhaps.
The 900MHz LO works just fine. It’s a simple oscillator with a 4-way clock splitter and terminating resistors so there’s not much that can go wrong but you never know. On another design, I might add a PI-pad to reduce the signal level down a bit because it’s pretty booming (~8dBm) but not causing any problems. I might also add a simple bandpass filter in there to attenuate the harmonics.
The DAC works great and will allow us to switch gains very fast. And it supports simultaneous updating of all four channels which is nice.
I did some preliminary phase noise measurements of the 870MHz LO and they look ok.
-81.5dBc/Hz at 10kHz offset is not bad considering my channel selectivity is 50kHz. I expect it to improve once the transmitter is handling the modulation and I can set the PDF for 500kHz. It should be around -90dBc/Hz so I’ll raise the PFD on Monday to see if it comes down.