A new board to experiment with undersampling technique.
“..If we use the sampling frequency less than twice the maximum frequency component in the signal, then it is called undersampling. Undersampling is also known as band pass sampling, harmonic sampling or super-Nyquist sampling. Nyquist-Shannon Sampling theorem, which is the modified version of the Nyquist sampling theorem, says that the sampling frequency needs to be twice the signal bandwidth and not twice the maximum frequency component, in order to be able to reconstruct the original signal perfectly from the sampled version. If B is the signal bandwidth, then Fs > 2B is required where Fs is sampling frequency. The signal bandwidth can be from DC to B or from f1 to f2 where B = f2 – f1. The aliasing effect due to the undersampling technique can be used for our advantage. When a signal is sampled at a rate less than twice its maximum frequency, the aliased signal appears at Fs – Fin, where Fs is the sampling frequency and Fin in the input signal frequency. “ from Why Use Oversampling when Undersampling Can do the Job? - Texas Instruments.
In an example case looking at FM band we sample the input 98 MHz with Fs = 56MHz and the aliased component will appear at 14 MHz ( 56*2 – 98).
As we know in advance that the signal is aliased, we can recover the actual frequency by using the N*Fs – Fin relationship. The undersampling technique allows the ADC to behave like a mixer or a down converter in the receive chain. For a band-limited signal of 98 MHz with a 20-MHz signal bandwidth, the sampling rate (Fs) of 56 Msps, the aliased component referred to 2*Fs will appear between 4MHz to 24 MHz (20 ±10 MHz.
An analog band pass filter is required at ADC input to avoid interference from other Nyquist band.
Undersampling Case of 98MHz Signal with 20MHz Bandwidth.
I designed a new breadboard with some modification. The PCB is named BBRF103 ver 0.5.
BBRF103 ver 0.5 - block diagram.
The J1 input uses a band stop filter for the FM Band 88-108 MHz.
This input is planned for experimental use within 120-500 MHz frequency range. Some specific band pass filter and LNA will be externally added for 50MHz, 144MHz or 432MHz band.
The FM Band Stop Filter - LTspice simulation.
The J2 input has a band pass filter for the FM band to analyze the full 88-108 MHz band spectrum at once. This filter is quite simpler as the FM signals are very strong versus possible interference.
FM band filter response.
Finally J3 is the HF input. The filter components values are changed from first version of BBRF103.
HF low pass filter.
The connector J4 is an optional input for an external reference signal. A capacitor must be mounted to enable this signal.
J5 and J6 are two programmable clock output from the Si5351 generator. May be used to synchronize external tuner oscillator.
RF switch type
I like to test a bi-stable subminiature DIP relay type HFD2/005-M-L2-D to switch RF instead of active switches.
The relay is a 5Volt dual coil latched one. I shielded it using adhesive copper tape that will be soldered to the PCB ground plane.
Relays used. The left one with a copper tape shield added.
The board scheme and PCB layout is at link http://www.steila.com/test/BBRF103_5.pdf
24/04/2019 - here http://www.steila.com/test/BBRF103_5B.pdf annotated scheme circuit to keep relays off during power on.
24/04/2019 - My prototype shows that the PCB layout around the PCB is worse and noisier than the previous PCBs. A makeover is needed.
email: ik1xpv AT gmail DOT com