MAX2606EUT part 2
So if you haven't followed the earlier blog there is an early attempt at breadboarding this device and its woes.
As we are dealing with RF there are some very special considerations and some which will not be immediately apparent. I am basically developing a Transmitter and a receiver in tandem and mixing this with digital.
Some of the important aspects to understand with the MAX2606. the choice of inductor is critical with a high Q. I have used both open and shielded inductors to tune. The later is very important. Adjustable core is essential.
Input signal to the MAX2606 is around 500mV. Exceeding the 700mV saturates the mixer and loses signal. Be warned!
This device is a voltage controlled oscillator, used as an FM transmitter. There are some aspects of FM transmission that need understanding to be of use.
So lets look at the situation so far.
1) I have had to change the coil type.
2) The receiver antenna matching has been improved.
3) The PCB layout has removed parasitic capacitance.
4) Capacitance values have been tightened to higher tolerance 5% or better.
5) The transmitter has been moved to Battery supply as the coupled USB connector was injecting noise into the circuit.
6) The supply to the transmitter has a local low pass filter.
7) Vtune supply voltage has a decoupler across to stabilize. In fact a better circuit would be to improve this voltage level regulation entirely.
Where am I ? I am still battling the transmission of data across the FM channel.
At present the transmitter circuit is isolated and fed from a signal generator, so that I have a clean source and a known voltage level.
I am certainly receiving signals. The range is as expected in the region of 100Hz to 30kHz. What I am noticing is some significant roll-off on the received signal.
1) The VCO is not an FM transmitter it has no pre-emphasis of the input signal and this should be 75uS for North America. This circuit will need trying out
2) I need to attenuate the signal to approx 500 - 600mV coming from the Microprocessor... Not essential at this time and we can do this with a passive filter circuit. My circuit board layout may become a lot simpler.
3) I am in stereo mode. I need to be in mono as the receiving circuit will be detecting the Stereo signal at 19kHz and splitting the output. This may be leading to the strange roll-off I am seeing in that the signal is filtered and split between left and right channels. This is a fairly quick software change.
To recap and the pictures are indicative of the signal between 100Hz and approx 20kHz
FM Source signal:
Signal Generator setting:
Received Signal:
As we are dealing with RF there are some very special considerations and some which will not be immediately apparent. I am basically developing a Transmitter and a receiver in tandem and mixing this with digital.
Some of the important aspects to understand with the MAX2606. the choice of inductor is critical with a high Q. I have used both open and shielded inductors to tune. The later is very important. Adjustable core is essential.
Input signal to the MAX2606 is around 500mV. Exceeding the 700mV saturates the mixer and loses signal. Be warned!
This device is a voltage controlled oscillator, used as an FM transmitter. There are some aspects of FM transmission that need understanding to be of use.
So lets look at the situation so far.
1) I have had to change the coil type.
2) The receiver antenna matching has been improved.
3) The PCB layout has removed parasitic capacitance.
4) Capacitance values have been tightened to higher tolerance 5% or better.
5) The transmitter has been moved to Battery supply as the coupled USB connector was injecting noise into the circuit.
6) The supply to the transmitter has a local low pass filter.
7) Vtune supply voltage has a decoupler across to stabilize. In fact a better circuit would be to improve this voltage level regulation entirely.
Where am I ? I am still battling the transmission of data across the FM channel.
At present the transmitter circuit is isolated and fed from a signal generator, so that I have a clean source and a known voltage level.
I am certainly receiving signals. The range is as expected in the region of 100Hz to 30kHz. What I am noticing is some significant roll-off on the received signal.
Here is the plan of attack:
Ideas:
2) I need to attenuate the signal to approx 500 - 600mV coming from the Microprocessor... Not essential at this time and we can do this with a passive filter circuit. My circuit board layout may become a lot simpler.
3) I am in stereo mode. I need to be in mono as the receiving circuit will be detecting the Stereo signal at 19kHz and splitting the output. This may be leading to the strange roll-off I am seeing in that the signal is filtered and split between left and right channels. This is a fairly quick software change.
4) Matching Output Impedance - Add a load of 32R.
5) Switch Output of receiver into high impedance mode through DHiZ - No idea bur worth an experiment to match the amplifier.
To recap and the pictures are indicative of the signal between 100Hz and approx 20kHz
FM Source signal:
Idea #4 The addition of the loudspeaker load - No effect. I didn't think it would but worth a try as it was quick and simple with a 36ohm load.
Idea #3 Programming in Mono Mode. Here the register switches from Stereo to Mono mode. The thinking being that some of the waveshaping is the through the stereo signal and only listening onto one channel. This in fact improved the signal but the 'Droop' is still there and limiting the amplifier in getting a good reproduction of the signal.
Idea #5 Is the DHiZ critical for connecting up and what does it do? The DHiZ seems to be another mute of the audio signal. No this had no effect.
Finally , it was a casual remark on base boosting that turned around my thinking. The 'BASE-BOOST' is in fact a total fiction. The RDA5807M actually places a band pass filter for the audio and takes out some lower frequencies giving a more treble signal. The base boost just adds the original signal back in ad provide no actual 'Base boost' In real terms its a bass reduction function.
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