Interesting choice of names AnyMajorDude, but thanks for your interest in the FM100B. There have been quite a bit of improvements made in the FM100B over the FM100. We listened to suggestions and tried some improvements to make the best possible performance at the price. I will try and list a few here for you and fellow BBS readers.
1. Improved audio low pass filter.
The old FM100 used switched capacitive filters for filtering out frequencies over 15kHz so that higher frequencies did not interfere with the stereo pilot. These types of filters are great for a lot of applications, but are poor for signal to noise ratios (snr) of critical audio gear. They can introduce spurious noise of their own due to the switching going on inside, as well as standard noise from the long chain of components from input to output. While the introduced signals are well above hearing, they did get introduced into the transmitted signal of the old FM100. Very little of it, but it was there.
I didn't like this added noise, so I re-designed the filter using some new software at our disposal. The neat part is the new filter is a notch-lowpass. This means it has both the characteristics of a notch filter and a lowpass. I designed the filter to have a notch at 19kHz and also at 38kHz for optimal rejection right at the pilot signals of the stereo multiplexer. These notches are complemented with a lowpass as well to reduce any frequencies above 15kHz at a rapid rate. We used precision resistors and capacitors in these sections to make the build simple and require no tuning on your part.
By using precision components, it also prevents phase problems near the cutoff frequencies of the filter. If one channel is slightly phase-shifted in respect to the other, the separation of the audio at your receiver suffers greatly. By maintaining phase coherence on the filters, separation remains excellent.
2. Added new feature of monitor output from the mixer.
When I ran my station in the past, I always had to have a receiver tied to a tape deck to record my shows, or to have a headphone output. This new line level output allows you to run directly into a tape deck, computer audio card for over the Internet transmission, or into our Stereo Headphone Amplifier kit so you can monitor your transmission without a separate receiver.
3. Improved modulator IC.
If any of you have used the FM25B, you will know what I am talking about. The newer versions of these chips have much better characteristics all around. For one, they include many functions right inside of the chip, like a limiter (clipper), pre-emphasis, and more. Also you no longer need to “tweak” a pot to adjust separation, it is hard-set inside of the part. In other words, you don’t need to adjust anything and separation remains excellent. If you can do a side-by-side with the FM100, the FM100B far outclasses the FM100 in audio quality.
Also due to the added limiter (clipper), you can get much more inherent volume on your station without over-modulation. You will need to adjust this to your tastes however, since a clipper by nature introduces distortion. Usually a good balance between volume and sound clarity is subjective, so I will leave it up to you.
Due to some other improvements, spurious RF signals are greatly reduced over those seen on the old FM100, which makes the FM100B much better for higher power operations overseas. I don’t have exact figures on this spec yet, but at least a 10dB improvement.
4. Better low frequency response.
In the interest of faster lock times on the FM100, the phase locked loop (PLL) was tuned to be a bit too fast. This translated to the PLL going fast enough to compensate for low frequencies in the audio, and not allowing it to be transmitted. (I wont get into PLL theory here). The new FM100B has a much slower loop, so you bass lovers will simply enjoy the improvement here.
5. Equipment-less tuning.
Now you no longer need a voltmeter, the display will now show you the PLL tuning voltage. This lets you instantly confirm proper setup and alignment, and also allow you to tune your VCO in the correct voltage range for frequency output.
I hope this answers your questions…
BTW, I had trouble hearing any hum in my receiver even with the volume turned up all the way. It is amazing what a metal case, double-sided board, and a good antenna will do for you! The signal to noise ratio of this baby is as good as if not better than many commercial stations!
[ April 12, 2002: Message edited by: Mark W ]