Rf and if digitization in Radio Receivers: Theory, Concepts, and Examples

The 6300 series sampling downconverters manufactured by Watkins-Johnson are an example of 

a technology potentially useful in the RF front-end of radio receivers. These sampling 

downconverters are microwave devices that use bandpass sampling techniques to downconvert 

an RF signal (typically in the 2- to 18-GHz range) to an IF signal (typically at 70 MHz up to 

500 MHz). In its current configuration, the sampling downconverter functionally performs in the 

same manner as the conventional mixer. However, the device is actually a sample-and-hold 

circuit that uses a step recovery diode (SRD) to generate a sampling pulse train from a sinusoidal 

“clock” frequency. The sample-and-hold circuits in these downconverters are designed to work 

in the 2- to 18-GHz range, far higher in frequency than the sample-and-hold circuits integral to 

most ADCs. In the future, this technology may be coupled with high-resolution quantizers to 

produce bandpass sampling ADCs with a high analog input frequency capability and the high 

resolution of quantizers that would operate at much lower frequencies. Use of these sampling 

downconverters for radio receiver applications requires some careful considerations, however. 

The frequency content of the input must be bandlimited properly to prevent spectrum overlap in 

the desired signal output as with any bandpass sampling scheme. These downconverters do 

suffer a higher conversion loss than conventional mixers. The maximum specified conversion 

loss for the 6300 series sampling downconverters can be anywhere from 13-25 dB depending on 

the specific model. Typical conversion loss for conventional mixers ranges from roughly 5-9 dB. 

As with any downconverter, spurious suppression must be considered carefully. The 

specifications for the Watkins-Johnson 6300 series sampling downconverters state a minimum 

15-dBc spurious suppression at a -10-dBm RF input level [36]. (This spurious suppression does 

not include the local oscillator (LO) signal leakage at the IF port or the 2

nd

 harmonic of the IF 

Operation at lower RF input levels will provide even better spurious suppression [37]. These 

types of devices may be very useful for future radio receiver front-ends and ADCs.  

It is tempting to compare the spurious suppression of the sampling downconverter to that of 

conventional mixers. While this comparison could be made for a specific mixer, it is not possible 

to make any general conclusions. While there are several types of conventional mixers, double-

balanced mixers are the industry standard [38]. Even within the double-balanced mixer type, 

there are several different classes of mixers. These classes of mixers include class 1, class 2, and 

class 3 mixers. Class 2 mixers require more LO power and have better spurious suppression than 

class 1 mixers. Similarly, class 3 mixers require more LO power and have better spurious 

suppression than class 2 mixers. The higher the mixer class, the more LO power is required but 

the better the spurious suppression is. In addition to the different classes of mixers, spurious 

suppression is a function of LO and RF input power levels and frequencies. While there are 

specifications on mixers that help predict spurious suppression, spurious suppression for a 

specific mixer should be determined by measurement over a well-defined set of conditions [38].  

 

38 

tải về 0.99 Mb.

Chia sẻ với bạn bè của bạn: