Background: This info is from a thread on the S40N/V50. These cars have antennas printed on the window, an active pre-amplifer, and a digital AM/FM receiver called the "AFM" (AM FM Module). So it doesn't apply as much to older Volvos with fixed antennas (240?) or retractable (850, S70, etc...), but most newer cars use this type of setup.
The original question was:
Quote, originally posted by christianwoodward » For those with modulators, do you still hear some FM stations that sound better than your modulator? I know those stations have some very expensive compression equipment but I was hoping to be close (mine sounds more "muddled" than some stations).
MrTippy asked me to post my answer here.
The AFM is a DSP (Digital Signal Processing) receiver that can lock onto just about any frequency, so the issue probably has less to do with the station's transmit hardware and more to do with the impedance between the factory antenna and your modulator's transmitting "antenna". Volvo's 9v battery connector snaps that connect the printed antenna to the balanced leads don't help the situation.
There is an active amplifier between the AFM (the box that demodulates the FM signal and transmits the digital data over the MOST fiber network) and the antenna. It's fed by a phantom voltage on the line. If I had to guess, the signal from the modulator is maxing out the gain on the active unit, and increasing the slew rate the FM receiver sees. To understand this, there are three important voltages to consider.
FM is frequency modulation, that is, a signal with a constant amplitude (voltage) and varying frequency. You can't measure the amplitude with a DMM because it's AC at a hundred megahertz, and the RMS function on a DMM works to around 2khz. The amplitude of an FM signal should have nothing to do with it's decoding, which is why FM signals sound better than AM over large/varying distances. The voltage coming off the antenna can only really be measured with an RF scope. It's very faint because it's coming from a station miles and miles away. So if you hook a DMM up to the factory antenna, it would read 0 volts.
In layman's terms, impedance is the AC version of resistance. It can't be measured with a meter because it's frequency dependent. There's a lot of complicated math (and stuff) behind it using complex numbers and such, but for this just assume that I'm right.
Most car FM antennas have a 50 ohm impedance 1/4 wave whip antenna and matching 50 ohm impedance antenna wire. This does not mean that if you took a DMM and hooked it to the antenna wire it would read "50 ohms". It would probably read close to 0. What it means is that the construction of the cable is such that the combine capacitive (between core and shield) and inductive (in the wire itself) elements will present a 50 ohm resistance to a 100 mhz signal.
Our cars don't have a 50 ohm 1/4 wave whip, instead the antenna is printed on the rear glass. That means that (1) there is a sensitivity compromise since it's likely 1/8 wave and (2) there is an non-standard impedance associated with it. The phantom pre-amp (will explain later) is responsible for turning this weak, mismatched signal into one that an OTS (off the shelf) FM demodulator can read reliably. The pre-amp doesn't expect a 50 ohm signal, and is not designed for one. That's why the hooking the FM modulator (50 ohm) up to the factory antenna terminals (something else) sounds muddled.
Phantom is the second kind of voltage, which means that there are only two wires (signal and shield) but there is a positive DC voltage that rides on the signal wire. The FM signal is still AC, but it is overlaid on top of a 12v DC signal. Rather than going between + and - through 0v, the signal goes from above 12 to below 12 (never below 0) and the "zero" voltage of the AC signal is 12 volts. This lets the pre-amp run off of the 12 phantom volts (with a little filter, no matter what the signal is at 100 mhz the average will be 12v) and still transmit the FM signal to the actual receiver. If you were to measure the voltage on the line connecting the AFM (module) and the pre-amp, the DMM would read 12v.
The third type of voltage is peak-to-peak, and refers to the amplitude of the signal coming out of the antenna/modulator. Since the printed antenna is less sensitive than a real quarter whip, the pre-amp is likely designed with a high gain (amplification) stage. In an ideal world, with infinite voltages in any polarity, this wouldn't matter for FM. But the car electrical system is +12 to 0, and it's hard to go much higher or lower than that. What ends up happening is the FM sine wave is clipped (cut off to a certain voltage) while the actual wave extends much higher then that. Since the receiver is digital, it works by detecting the voltage peaks of the signal at a specific frequency. Since the signal is clipped, the peak occurs before it should. I won't go into heterodyne receiver theory, but this poses an upper cap on how much higher the frequency of the signal can deviate from the base frequency before the whole thing blends into a solid "1". In other words, you loose treble.
The ideal solution would be to rig the FM transmitter to run off of the phantom voltage coming from the AFM, and output directly to the 50 ohm input on the AFM. This would eliminate any feedback loop issues, impedance issues, and clipping issues simultaneously. If I get some free time and can find an old FM transmitter I'll play around with it. It shouldn't be too hard, in theory anyway....
Hope that helps clear things up
As it turns out, many of the S40-V50 with *basic* audio pre-2007 (like my 2005) do not have an AFM, but rather the radio receiver is built into the CD player. For these cars, consider adding a tap manually (https://forums.swedespeed.com/showthr...system./page10)