Q. Can you suggest a method of implementing an AGC with the CMX980A or CMX981?
A. The following text illustrates one method of Automatic Gain Control or AGC, a method employed to ensure that the optimum received signal is used when receiving and decoding a signal. A received signal level will be very much higher when the receiver is located adjacent to a base station than it would be situated 2 miles from a mobile unit.
Hardware:
Figure 1 is a simple illustration of one method of AGC (Automatic Gain Control).
The Figure 1 shows a low noise, linear VGA (Variable Gain Amplifier) is used in the IF of the radio section.
This is driven by one of the onboard CMX980A DACs and updated regularly when receiving. The VGA will be used for coarse level adjustment, prior to down converting too Baseband.
Its main purpose is to ensure the CMX980A ADC’s are not saturated and the signal distorted.
Internally the Gain and Offset block will also be used to offer smaller, higher rate adjustments to incoming signal level. This unit’s purpose is to fine-tune the incoming signal to make full use of the IQ decoder.
The received IQ amplitude data is sent to the local Microcontroller/DSP that performs the final decoding and for calculating the required “on-the-fly” adjustments to the two-stage AGC.
Figure 1, a block diagram of the CMX980A receiver’s path and AGC implementation.
Software
Figure 2, represents the received constellation diagram as perceived by the controlling Microcontroller/DSP. For decoding, the 8 constellation points that make up a good Pi/4DQPSK signal are used to calculate the received 3bit symbols by location in the diagram.
The purpose of AGC is to ensure that at all times the 8 IQ points remain stable and in a similar position no matter where the radio receiver is located or the amplitude of the incoming signal. If the signal amplitude increases in strength the 8 points will move outwards perpendicular to the centre position, if the signal decreases the points move towards the centre. See Figure 2, blue constellation point.
To try an maintain a reliable decode of the IQ points, thresholds can be set up that react to the position of the IQ points.
In figure 2 the minimum threshold is marked in red and the maximum in green.
In this document they will be referred to as min_limit and max limit.
Figure 2, I and Q constellation diagram
By taking a period T (where T > Symbol rate) the samples that make up the 8 IQ points are averaged. If the mean is higher than max_limit then the receive gain will be reduced. If the mean is lower than min_limit then the receive gain is increased.
It is important to use average (mean) values to ensure that the gain feedback loop bandwidth is high and that the incoming signal is not artificially modulated. Whatever period T is chosen it will be ultimately limited by the refresh rate of the CMX980A DAC and gain blocks.
It is intended that the VGA be used as a gross gain element that is updated periodically when there is consistent movement in received signal amplitude.
The CMX980A gain and offset block is intended for smaller but more frequent gain adjustments.
This method can also be used to update the offset block in the CMX980A by ensuring that the sum of the IQ points are equal to zero.
It is important to note that the decoder algorithm being used must be aware of the adjustments being made to the received signal and compensate for it when calculating symbol values and consequently decoding data.
In summary:
IQ signal is greater than max_limit (mean value over period T1) then reduce CMX980A gain block value.
If T1 remains positive for period T2 then reduce VGA.
IQ signal less than min_limit (mean value over period T1) then increase CMX980A gain block value.
If T1 remains negative for period T2 then increase VGA.
Additional hysteresis should also be included to decrease ripple and improve SINAD.
Note: T2 >> T1
Q. I see that I and Q methods of generating and receiving baseband signals for wireless data devices is popular with CML, can you direct me to sources of I/Q hardware; for example I/Q mixers?
A. There are many manufactures of semiconductor solutions for I/Q radios, a comprehensive list would be difficult to maintain however the first companies you should consider visiting are as follows:
| Functions | Manufacturer | Link |
| IF AGC amplifiers, I/Q mixers, PLL’s, Modulators and demodulators | ||
| Maxim | http://www.maxim-ic.com | |
| Analog Devices | http://www.analog.com | |
| RFMD | http://www.rfmd.com/ | |
| Atmel | http://www.atmel.com/ | |
| Infineon | http://www.infineon.com | |
Please note the CML disclaimer notice for applications and FAQ information.