Radio frequency tracking filter

Details Radio frequency tracking filter Radio frequency tracking filter

1998-03-23

2002-09-17

Bost, Dwayne (Department: 2681)

Telecommunications

Receiver or analog modulated signal frequency converter

With particular receiver circuit

C455S302000, C455S307000, C455S340000, C333S176000

Reexamination Certificate

active

06453157

ABSTRACT:

FIELD OF THE INVENTION
The present invention generally relates to filters, and in particular, to a tracking filter that tracks received signals over a relatively wide radio frequency (RF) band.
BACKGROUND AND SUMMARY OF THE INVENTION
Electrical wave filters are used in communication systems to control, distinguish and/or separate certain frequency bands of an electrical signal from other signals in neighboring frequency bands. A filter is typically placed between two terminal pairs or two ports of an electrical circuit in order to modify the frequency components of a signal. Ideally, a filter will allow a certain band or bands of frequencies of a signal to pass through it with little or no attenuation. These are called the passbands of the filter. Other bands of the same signal are substantially attenuated. These bands are called the stopband or stopbands of the filter. Filters can be classified as to the range of frequencies that are passed and those that are attenuated. The more common classifications of filters are lowpass, highpass, bandpass, and bandstop filters. The present invention pertains particularly to a bandpass filter whose passband is in the radio frequency range and whose passband can be shifted in frequency electronically.
In the practical case, the band of frequencies separating a passband from its adjacent stopband or stopbands is called a transition band. The cutoff frequency of a filter defines the passband limit and usually corresponds to a frequency that is 3 dB down from the passband transmission maximum. Lowpass and highpass filters have only one cutoff frequency, while bandpass and bandstop filters have two cutoff frequencies.
If the frequency components of a signal falling within the passband of an ideal filter are applied to the input of the filter, they are “passed” by the filter without attenuation to the output of the filter. Those frequency components in the stopband of the ideal filter are suppressed, and the transition band or bands have zero width. However, the amplitude of the frequency components falling within the passband frequency range of a practical RF bandpass filter may experience attenuation due to circuit absorption, reflections, radiation, and the “roll-off” to the 3 dB cutoff frequencies at the passband edges. All of these contributions to signal attenuation result in power loss of the signal passed to the output of the filter.
A filter attenuation or insertion loss function A(&ohgr;) may be defined as the decrease in power delivered to the load when a filter network is inserted between the source and the load:
A
(&ohgr;)=10 log
10
(|
P
max
|/|P
out
|),
where &ohgr; is radian frequency 2&pgr;f, P
max
is maximum power available to the load, and P
out
is the power delivered to the load when the filter is between the source and load. In general, the insertion loss at the minimum passband attenuation is often called “flat loss.” The definition of insertion loss applies at any frequency and includes the attenuation due to the filter's selectivity.
Two major problems with tracking filters include a large value of flat loss or substantial changes to the filter's flat loss over the tracking range or both. For example, a large value of flat loss for an RF receiver tracking filter causes the desired received RF signal to be substantially attenuated regardless of where it falls in the bandpass and adversely affects the gain distribution, the sensitivity, and the noise figure of the receiver. The change in flat loss of the RF tracking filter (and therefore the change in the passband insertion loss) over the tracking range causes another problem in the distribution and control of gain through the receiver which changes the filter's noise figure over the tracking range. This latter problem makes certain receive bands more noisy and less desirable than other receive bands. Variation in the flat loss of the tracking filter and/or the variation in the passband insertion loss of the tracking filter over the tracking range may cause a decrease in receiver sensitivity when the passband of the tracking filter is in one position as compared to another position in the RF tracking range.
Most radio receivers are of the superheterodyne or double superheterodyne type in which the incoming RF signal is fed to a mixer and mixed with a locally-generated signal from a local oscillator. The mixer output includes an intermediate frequency (IF) signal equal to the difference between the locally-generated signal and the received RF frequency but still containing all of the original modulation. Stable high gain amplification and greater selectivity are directly attributable to the use of the intermediate frequency.
Many receivers employ “high-side injection” because the required voltage-controlled oscillator (VCO) tuning range for the synthesizer providing local oscillator frequency for the first mixer in the receiver chain is smaller (percentage-wise) for high-side injection than for low-side injection. When high-side injection is used, the problem is to design a bandpass-tracking filter of small size using practical passive element (i.e., inductors and capacitor) values. A passive filter requires no power for signal passband gain, but a control voltage may be used to change the position of the filter passband. Also, the passband of the tracking filter should have a low insertion loss and a relatively constant insertion loss as it is shifted in frequency over a relatively wide tracking range which can encompass the entire RF receive passband of the radio. In addition, the filter should have substantial attenuation on the high side of the passband.
Typically, achieving small size is associated with the simplicity of the filter (i.e., the order of the filter). On the other hand, substantial attenuation on the high side of the passband is associated with more filter complexity. Added complexity is especially a problem in achieving substantial attenuation on the high side of passband of the filter close enough to the passband to attenuate three of the most potentially troublesome and unwanted signals which may be received by the radio at f
RF
+(½)f
if
, f
RF
+f
if
, and f
RF
+2f
if
, commonly called the half-IF, the IF, and the image spurious signals, respectively. These unwanted RF signals may be produced directly (in varying degrees) in the IF band in the IF section of the receiver due to mixer nonlinearity, straight IF frequency pickup, and normal mixer action, respectively. If the RF tracking band to be covered is wide, then one or all of these unwanted signals may fall within this RF tracking band when the desired RF signal is received at the lower frequency portion of the RF tracking band. Therefore, a fixed passive RF filter with a passband corresponding to the width of the RF tracking band cannot be used because the unwanted signals would not be attenuated. However, if a narrower filter is used, then a desired RF signal received in the upper section of the RF tracking band would be attenuated. A tracking filter is needed to pass the desired RF signal received on the lower frequency section of the RF tracking band while still attenuating three spurious signals described above, one or all of which may occur within the overall RF tracking band.
One possible solution to deal with such image frequencies is to employ several passive filters, such as surface acoustic wave (SAW) filters, having different center frequencies and, using switches, select one of several filters based on the received signal frequency. The drawback with this solution is that multiple filters and switches corresponding increase the cost, complexity, and size of the radio receiver. Therefore, what is needed is a single passive bandpass filter (small size and low complexity) that can be manufactured inexpensively using realistic element values which (1) can be easily controlled to operate over a relatively wide RF tracking band, (2) has relatively low and constant insertion loss over that RF tracking band, (3) substant

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