An amplitude modulation/phase modulation (AM/PM) conversion coefficient ([K.sub.p]) is known to produce distortion products from AM, but it can also convert in-phase noise into quadrature noise and cause system degradation. This article discusses an accurate means of measuring [K.sub.p]. A frequency effect was found when measuring [K.sub.p] near the carrier. An equation is derived showing the noise degradation caused by this conversion. Another effect is also discussed where an offset carrier in a channel can produce a noise plateau located symmetrically about the carrier.
The AM/PM conversion coefficient [K.sub.p] is defined as the ratio of the change in phase shift in degrees to the change in input drive in decibels. Therefore, any transmission deviations causing AM will be converted to PM by [K.sub.p], including noise. The worst [K.sub.p] occurs near saturation. The degradation that occurs there is important when constant envelope signals are operated at saturation, such as analog television frequency-division multiplex/FM, and some near-constant envelope data signals using phase detection. [K.sub.p] should not be confused with the AM/PM transfer coefficient, which transfers AM from one carrier to PM on another carrier in the same channel.
[K.sub.p] was found to not only produce distortion, but also to increase the quadrature thermal noise by converting in-phase noise to quadrature noise. This noise conversion will degrade any transmission system but is important particularly to satellite systems where increases in power are costly. The overall satellite link performance at saturation will be degraded if the limiters in the spacecraft and the earth station do not have small [K.sub.p]s. Limiters in earth station equipment have AM/PM conversion coefficients of [less than] 0.1 [degree]/dB. For equal uplink and downlink carrier-to-noise ratios (CNR) and a perfect limiter in the earth station, the degradation for a [K.sub.p] of 6.6 [degrees]/dB and 3 [degrees]/dB is 1.7 and 0.4 dB, respectively. The degradation would be 3 dB if the uplink CNR dominates. One reason why this condition was not apparent in the past was that the CNR at the output of a perfect limiter is 3 dB better than the CNR of a linear amplifier. In a sense, the in-phase noise is suppressed, leaving only the quadrature noise. This result is important in understanding some of the other results discussed in this article.
Measurements showed that the degradation was real and not a test setup problem. No literature was found to explain this problem. Finally, the degradation vs. AM/PM conversion coefficient was plotted for several channels and a strong correlation was found. At points that did not correlate well, the method of measuring [K.sub.p] was suspected to be at fault. Later, this theory was proved to be true.
A block diagram of one of the transponders in the hardware simulator is shown in Figure 1. The-receiver amplifies and translates the uplink band to the downlink band and the channel filter separates the desired channel from the other channels. A flux control attenuator sets the desired gain of the channel to saturate the traveling-wave-tube amplifier (TWTA) at the desired uplink...