To complete the characterization of the Mini Circuits UFC-6000 RF Frequency Counter to be used for beat note measurement as a part of frequency offset locking loop. The aim of this setup was to obtain the bode plots and PSD plots for the FC.
Detail about the Setup:
UFC RF Frequency Counter: Described in detail in one of my previous elog (http://nodus.ligo.caltech.edu:8080/40m/10020)
Raspberry Pi: Raspberry Pi will be running Raspbian which is a version of Linux, and not a RTOS. When sampling data at a certain frequency we want samples to occur at fixed time intervals corresponding to the sampling period. A normal operating system cannot provide us with this functionality, and there will be jitter (variation) in the time difference between consecutive samples. Whether this is an issue depends on how much jitter we have and what the specific application is. In our application(measuring phase and noise), the jitter has to be taken into consideration. Hence for data acquisition we need to sample with much more tightly defined sampling periods (reduced jitter) which can be done by providing an external timing standard(Like a square pulse of the frequency same as the sampling rate of the FC ).
ADC : The ADC serves for two different conversion processes in the setup:
1) For converting modulating analog signal(from SRS 30 MHz Wave Generator) into digital signal for data analysis on Raspberry Pi.
2) To provide an external clock reference to the Raspberry Pi.
Interfacing ADC(ADS1115) with Pi:
In order to set the modes of operation defined above we must set the config register within the ADS1115. A register is simply a memory location within the chip. Registers are made up of bytes (8 bits) of data. Registers are typically either one or two bytes long. The bits are:
Bit  This bit is used to start a conversion, by setting this bit to 1 a conversion is initiated. When reading the config register this bit remains equal to 0 while the conversion is carried out, and is set to 1 once the conversion is complete, we can monitor this bit to find the status of a conversion
Bits [14:12] These bits set which pin to use as input to the ADC. Note that we can choose either single ended or differential mode through setting these bits. Note that each configuration has two inputs AIN~p~ and AIN~n~. By setting AIN~n~ to GND we obtain a single ended input with AIN~p~ as the input.
Bits [11:9] These bits set which setting of the programmable gain amplifier to use
Bit  Continuous conversion / No Continuous conversion
Bits [7:5] Set the samples per second (sps) value
Bit [4:2] Comparator setup, we will not use the comparator so these bits are irrelevant
Bit [1:0] Comparator mode, set to 11 to disable the comparator.
Four channels are used in differential mode for A-D conversion of two analog signals, one the slow modulating signal input and the other for a square signal of 10 Hz (same as sampling rate of FC(0.1 s)).
The raspberry Pi reads the external trigger from ADC and starts reading input from the FC only when the square signal is 1. Thus in this way we can avoid the clock jitter and timing can be as accurate as the RTOS.
Three function generators are used in the setup:
The setup is attached as pdf. The computer scripts will follow this elog.
The input and output modulated signals are recorded and the delay and noise of the FC are to be estimated.