On 7/29/20 1:11 PM, KENT BRITAIN wrote:
Got a Spectrum Analzyer??
Would be very interesting to see where the strongest signals are.
Especially if the coax run is looking like a good end feed antenna
in the AM broadcast band.??? Considering their bridge circuit, I
wonder just how much reject the nano's have of 50-60 Hz? coupled
signals from the AC mains?
The bridge is just a group of resistors, R9,10,11, and 12.
however, what you're really interested in is the filtering components that go into the mixer RF input. and that's 100nF into the input impedance of the SA612AD. The data sheet gives that as 1.5k and 3pF in parallel up to 50 MHz.
At power line frequencies, 100nF is about 26k, so it's a 17:1 voltage divider (ignoring the 3pF, which is infinite at 60Hz), so, about 40 dB rejection.
At broadcast band, 1 MHz, though, there's not much attenuation and the signal passes pretty well. But there, you're relying on the selectivity of the filter after the mixer. As long as the mixer doesn't saturate, you're ok. The IP3 on the mixer is pretty low (-13dBm) and the mixer starts to compress at about -30dBm. (Of course, maybe you want compression and harmonics.. that's how you get a 200MHz part to work at 900 MHz, after all)
The bridge output to the U7 mixer has a 390:50 voltage divider, which knocks it down about 19dB. So, at a first guess, the receiver mixer starts to compress at -10dBm on the bridge.
If your antenna has 100s of millivolts of AM BCB interference with a 50 ohm load on it, you're going to have problems - 1mVrms is -47 dBm, and still in the linear region. 10mV is -27 dBm and getting close. 100mV and you're definitely living dangerously.
The Port 1 input has a voltage divider of 50:480, about -20.5 dB.
BTW, hooking up a high z meter or scope to an antenna doesn't tell you much about what voltage you'll see with a load on it. The antenna's source impedance well away from resonance is probably pretty high (mostly reactive, but that still forms a voltage divider with the 50 ohm load)
I think this topic will be a nice addition to my next antenna column.
This is always a problem with using a VNA to look at active circuits,
or in this case, active antennas.
Kent WA5VJB
On Wednesday, July 29, 2020, 3:04:50 PM CDT, Tyler <twardnw@...> wrote:
Kees : I had the tuner disabled when running the SWR sweep from the transceiver.
Ted/Kent : I'll look into buying/building a LPF for this, see if I can get some better rejection ahead of the NanoVNA
