Wideband radio-frequency broadcast recorder and radio testing system

A tape recording apparatus in the form of a modified VCR is employed to record broadcast signals received by an antenna while moving through a radio-broadcast reception area. The recorded signals are maintained in an undetected, modulated-carrier form so that played-back signals from the VCR can be coupled to a radio receiver in a fixed location for testing under mobile conditions. A commercially available VCR is modified to avoid modulating FM signals by frequency a second time.

Background of the Invention 
The present invention relates, in general, to a method of providing test 
signals in a fixed location corresponding to radio-frequency broadcast 
signals received by a moving receiver and, more specifically, to a tape 
recorder for storing modulated radio frequency signals received from a 
plurality of antennas on a moving vehicle. 
In developing a product, proper engineering design requires that testing 
and verification be done in the actual environment for which the product 
is intended to be used. In the case of automobile radio receivers, this 
means expensive, inconvenient, and time-consuming road tests. Furthermore, 
the kinds of testing that can be done in a mobile vehicle are limited. It 
would be advantageous to simulate road conditions in a fixed location 
where radio performance can be better analyzed. 
Road testing is particularly critical with diversity reception systems 
which employ a plurality of spaced apart antennas. Spaced receiving 
antennas receive signals that have traveled over different transmission 
paths from a transmitting tower. The antenna separation in a diversity 
reception system is such that each received signal is subject to different 
propagation conditions, including undesirable conditions which may cause 
fading or interference from reflections. Therefore, the various signals 
received from the antennas exhibit different received signal strengths. A 
diversity radio receiver selects the highest quality received signal, or 
combination of signals, for detection of the audio content that is then 
amplified and output to a speaker. Since transmission paths change 
continually in a moving vehicle, mobile testing is unavoidable in 
designing diversity systems. 
U.S. Pat. No. 4,713,801 shows an automotive tape recorder for recording a 
radio broadcast after detection in a radio tuner. Thus, the recorded 
information is of the audio output and not the input antenna signals that 
were received by the radio tuner and processed to obtain the audio output. 
Therefore, this recorder would be adaptable only for testing of an actual 
radio tuner output after detection of audio content. 
Video recorders have been used in conjunction with radar receivers to 
record radar signals, as shown in Miller, U.S. Pat. No. 4,047,170 and 
Schwab et al, U.S. Pat. No. 4,287,533. In each patent, radar information 
is formatted for recording with commercial video tape recorders. The 
information can then be played back in order to reproduce the radar system 
output. However, a working radar system is needed in order to either 
record or play back radar information. Therefore, these systems are not 
useful for testing of either mobile or fixed systems. 
SUMMARY OF THE INVENTION 
Accordingly, it is a principal object of the present invention to provide 
method and apparatus for efficiently analyzing automotive radio 
performance. 
It is a further object of the invention to record radio-frequency broadcast 
signals from at least one moving antenna in order to provide test signals 
in a fixed location to aid in the development of automotive radios and 
radio circuits. 
It is another object of the invention to modify a commercially available 
video cassette recorder in order to record undetected broadcast signals 
from an antenna. 
It is still another object to record and playback a plurality of signals 
having the same frequency with a single recording media. 
These and other objects are achieved in a method of providing test signals 
in a fixed location that replicates radio-frequency broadcast signals 
received by a moving receiver. The method comprises the steps of: moving a 
vehicle through an area receiving actual broadcast signals; coupling RF 
broadcast signals from an antenna mounted on the vehicle to a recording 
device; recording the broadcast signals on a recording media; transporting 
the recording media to the fixed location; and playing back the recorded 
broadcast signals as the RF input to a radio under test. Preferably, the 
broadcast signals may be derived from a plurality of antennas on the 
vehicle, and the signals from each antenna may be distinctly recorded by 
the recording device. 
Another aspect of the invention is an apparatus for recording and 
reproducing undetected (i.e., modulated carrier) radio-frequency broadcast 
signals in a broadcast band. The apparatus comprises input means adapted 
to be coupled to an antenna for receiving broadcast signals in a first 
range of radio frequencies in the broadcast band. First frequency 
conversion means is coupled to the input means for mixing the broadcast 
signals with the first oscillating signal to generate frequency-shifted 
signals in a second range of frequencies coinciding with a recording 
frequency band of a recording means. The recording means is coupled to the 
first frequency conversion means for recording the frequency-shifted 
signals when in a record mode. A recording means is also used for 
reproducing the frequency-shifted signals when in a playback mode. Second 
frequency conversion means is coupled to the recording means for mixing 
reproduced frequency-shifted signals with a second oscillating signal to 
replicate the original broadcast signals in the broadcast band received at 
the antenna.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, an automobile 10 is shown including a diversity 
reception system 11 which includes spaced apart antennas 12 and 13. A 
radio transmitting tower 14 transmits modulated-carrier radio-frequency 
(RF) signals 15 through the atmosphere, as shown, to the locations of 
antennas 12 and 13. 
Typically, RF signals 15 arrive at antennas 12 and 13 by line-of-sight 
transmission and by reflection from obstacles, such as hills and buildings 
(exemplified in FIG. 1 as a building 16). In AM transmission, reflection 
from the ionosphere is also important. Due to localized changes in the 
ionosphere, radio signals received by a single antenna are subject to 
fading, which is a drift in the level of received RF signal strength. 
Likewise, constructive and destructive interference due to reflections 
from obstructions cause variations in received strength of FM signals in 
moving vehicles. The signals received at antenna 12 and antenna 13 will 
often times have different signal strengths due to their separation and 
the fact that the signal paths between RF transmitting tower 14 and each 
antenna are independently affected by reflection and fading. 
Diversity reception system 11 either selects the best signal at any one 
time for audio processing or produces a combined signal with relative 
weighting according to the respective signal strengths. 
Conventional testing of diversity reception system 11 requires road testing 
in order to obtain input signals from antennas 12 and 13 which have the 
characteristics expected during normal operation in a vehicle. Such field 
testing requires travel to remote locations so that a wide spectrum of 
conditions can be experienced. However, it may be inconvenient or 
impossible to conduct simultaneous or repetitive analysis within the 
confines of the vehicle as the vehicle is moving. 
The problems of the prior art testing methods are solved, according to the 
present invention, by substituting a recording device for diversity 
reception system 11 of FIG. 1. The antenna signals are coupled to the 
recording device and are recorded without being detected, although the 
signals can be frequency shifted in order to conform to the bandwidth of 
the recording device. The recording media can then be transported to a 
fixed location for testing radio equipment. The recorded signals are 
reproduced in a form substantially equal to the broadcast signals in a 
modulated-carrier form and are coupled to the input of the radio receiver 
to be tested. 
Referring now to FIG. 2, an apparatus for practicing the present invention 
is shown wherein signals from diversity antennas 12 and 13 are provided to 
a video cassette recorder (VCR) 25. VCR 25 has a characteristic frequency 
range within which signals may be recorded on a magnetic tape contained in 
a video cassette. For example, the S-VHS type of VCR has a frequency range 
of from 1.5 megahertz (MHz) to 6.5 MHz. The frequency range of VCR 25 does 
not match the AM or FM broadcast band of RF signals received by antennas 
12 and 13 (AM is broadcast from 535 KHz to 1.605 MHz and FM is broadcast 
from 88.1 MHz to 107.9 MHz). However, because the diversity signals from 
antennas 12 and 13 are desired to be recorded separately and distinctly by 
VCR 25, means are provided between the antennas and the VCR for converting 
the frequencies of RF signals received by the respective antennas to 
frequencies that are within separate portions of the frequency range of 
VCR 25. AM signals must be converted up to the frequency range of VCR 25 
while FM signals must be converted down in frequency. 
An up or down converter 20 has its input connected to antenna 12 and its 
output connected to a first bandpass filter 21. A second up or down 
converter 22 is connected to antenna 13 and to a second bandpass filter 
23. A summing amplifier 24 receives the outputs from bandpass filters 21 
and 23 and provides a summed output to the recording input of VCR 25. 
Separate portions of the recording frequency range of VCR 25 are dedicated 
to recording signals from antenna 12 and antenna 13, respectively. For 
example, down converter 20 and bandpass filter 21 convert a band of FM 
frequencies received on antenna 12 to a first portion of the tape 
frequency range and down converter 22 and bandpass filter 23 convert 
signals of the same FM frequencies received on antenna 13 to a second 
portion of the frequency range of VCR 25. 
Recorded signals on a recording media can be reproduced and converted into 
appropriate test signals for a radio receiver using the apparatus shown in 
FIG. 3. A playback output of VCR 25 is coupled to bandpass filters 30 and 
32 which have substantially identical frequency characteristics as 
bandpass filters 21 and 23, respectively. A down or up converter 31 
couples bandpass filter 30 to an antenna input A of a radio receiver 35 
and a down or up converter 33 couples bandpass filter 32 to an antenna 
input B of receiver 35. Converters 31 and 33 restore the recorded RF 
signals to the broadcast band originally received by antennas 12 and 13, 
whereby appropriate test signals are provided to the diversity inputs of 
receiver 35. Specific details of the frequency converters and bandpass 
filters are within the knowledge of those skilled in the art. 
In order to record a substantial portion of a broadcast frequency band 
simultaneously, components 20-23 and 30-33 must be operable over a 
relatively wide bandwidth, such as 2 MHz in bandwidth for providing 
signals to be recorded in one-half of the pass band of VCR 25. Thus, a 2 
MHz portion of the FM frequency band from 88.1 to 107.9 MHz can be 
recorded in diversity using a VCR 25 having a frequency range of 5 MHz 
while maintaining a 1 MHz wide deadband between channels for signal 
isolation. 
Image rejection is achieved while operating on a wide bandwidth, by 
employing the converter structure shown in FIG. 4. It is well-known in the 
art that when a mixer is used to shift a signal in frequency, both a 
desired RF frequency and an image RF frequency signal appear in the output 
of the intermediate frequency (IF). These two RF frequency signals 
correspond to the sum and difference frequencies of the mixed signals. In 
a typical radio receiver, image frequencies can be rejected using a 
restricted bandwidth in a tuned RF amplifier stage. However, due to the 
desire to simultaneously record multiple broadcast stations in the present 
invention, relatively wide bandwidth components are used. Therefore, it is 
not feasible to reject image signals at the RF input. Instead, a converter 
40 is preferably comprised of two or more mixing stages such that the 
image frequencies are located outside the FM band in a substantially 
unused portion of the very high frequency band, e.g., around 110-150 MHz. 
Converter 40, shown in FIG. 4, can be either an up converter or a down 
converter depending on the particular broadcast frequency range being 
recorded or reproduced. An incoming signal, which may be either an RF 
signal from an antenna or a signal at the tape frequency from the VCR, is 
provided to a wideband mixer 41 which also receives a mixing signal at a 
first predetermined frequency f.sub.1 provided by a first oscillator 42. A 
resulting IF signal is output from mixer 41 through an IF bandpass filter 
43 to a second wideband mixer 44. Wideband mixer 44 receives a mixing 
signal f.sub.2 from an oscillator 45. Wideband mixer 44 provides an output 
signal in either the tape frequency range during recording, or in the 
broadcast band of the originally recorded signal during playback. That 
output signal is provided either to a filter prior to recording, or to a 
radio during playback. 
FIG. 5 shows a detailed embodiment of the invention for recording FM 
broadcast signals in diversity. A pair of antennas 12 and 13 receive 
broadcast signals and are associated with a pair of channels respectively 
designated as A and B. Channel A includes the series connection of a down 
converter 50, a bandpass filter 51, and a down converter 52. Local 
oscillators 56 and 57 are connected to converters 50 and 52, respectively. 
Down converter 50 receives a mixing signal from a local oscillator 56. The 
frequency generated by local oscillator 56 is controlled by an operator 
input command designated f.sup.*. For example oscillator 56 may be a 
voltage-controlled oscillator and the operator may provide f.sup.* by 
means of a voltage signal from a potentiometer. Bandpass filter 51 
receives IF signals from converter 50 and outputs a signal bandwidth of 2 
MHz centered on about 10.5 MHz. 
Local oscillator 57 provides a constant 8 MHz mixing signal to down 
converter 52. The 10.5 MHz centered IF signal from bandpass filter 51 is 
then frequency shifted by down converter 52 to about a 2.5 MHz centered 
signal. 
Channel B includes the series connection of a down converter 60, a bandpass 
filter 61, and a down converter 62. Down converter 60 receives a mixing 
signal at frequency f.sup.* from oscillator 56. A local oscillator 63 
provides a 16 MHz mixing signal to down converter 62. IF signals from the 
output of down converter 60 are limited to a bandwidth of about 2 MHz by 
bandpass filter 61 and are frequency shifted down to a center frequency of 
about 5.5 MHz by down converter 62. 
The frequency-shifted signals from down converters 52 and 62 are provided 
to a summing amplifier 53. The summed output signal is a combination of 
the channel A and channel B signals. The combined signals are coupled to a 
low-pass filter 54 which has a cutoff frequency of 6.5 MHz to remove any 
image signals outside the tape frequency. The output of low-pass filter 54 
is provided to the record input of a modified VCR 55. Thus, a 2 MHz wide 
portion of the FM band is recorded in diversity on modified VCR 55. 
FIG. 6 shows the frequency shifting of signals during processing by the 
circuit of FIG. 5. The specific portion of the FM band recorded is 
determined by the commanded value of frequency f.sup.* generated by 
oscillator 56 and mixed with the A and B channels as shown in FIGS. 6a and 
6b. Down converters 50 and 60 provide frequency-shifted broadcast signals 
with a range of intermediate frequencies centered on about 10.5 MHz 
containing the desired portion of the FM band without any significant 
contribution from image signals as shown in FIGS. 6c and 6d. Down 
converters 52 and 62 provide further frequency-shifting of the A and B 
channels to different portions of the frequency range of VCR 55, where 
they are respectively centered on about 2.5 MHz and 5.5 MHz, as shown in 
FIG. 6e, with an 8 MHz bias signal added (further explained in connection 
with FIG. 7). Down converter 52 receives an 8 MHz mixing signal from 
oscillator 57 to produce a 2 MHz wide band of frequencies centered on 2.5 
MHz. Likewise, down converter 62 receives a 16 MHz mixing signal from 
oscillator 63 to generate signals centered on about 5.5 MHz. 
Returning to FIG. 5, apparatus for reproducing channel A signals include 
the series connection of a low-pass filter 64, an up converter 66, a 
bandpass filter 67, and an up converter 68. An oscillator 71 provides 8 
MHz mixing signals to up converter 66. An oscillator 70 provides mixing 
signals at a frequency f.sup.* to up converter 68. Channel B signals are 
reproduced by the series connection of low-pass filter 64, an up converter 
76, a bandpass filter 77, and an up converter 78. Up converter 76 receives 
16 MHz mixing signals from an oscillator 79. Low-pass filter 64 removes 
the 8 MHz bias signal used in recording and any noise present outside the 
range from 1.5 to 6.5 MHz used to record the FM broadcast signals. Up 
converter 78 receives mixing signals at frequency f.sup.* from oscillator 
70. 
Up converter 66 reproduces intermediate frequency signals in a range 
centered on about 10.5 MHz as shown by FIGS. 6f and 6h. Bandpass filter 67 
has a pass band that is 2 MHz wide centered on 10.5 MHz, which thereby 
provides up converter 68 the desired IF signals. The IF signals are 
converted to the FM band according to the frequency received from 
oscillator 70, as shown by FIGS. 6h and 6j. The commanded frequency 
f.sup.* can have a value such that the FM signals in channel A can be 
placed anywhere within the FM band, including the portion from which the 
signals were originally recorded (i.e., when f.sup.* to the value used 
during recording). In channel B, up converter 76 provides an identical IF 
frequency band centered on 10.5 MHz by virtue of oscillator 79 providing a 
16 MHz mixing signal (FIGS. 6g and 6i). The channel B IF signal is shifted 
to the FM band (FIGS. 6i and 6k) by up converter 78, which receives a 
mixing signal at frequency f.sup.* from oscillator 70. The FM outputs of 
channels A and B thus provide RF broadcast signals substantially identical 
to the RF signals originally received in diversity for recording. 
FIG. 7 shows the modification to VCR 55. A record input is coupled to a 
buffer 80 for providing electrical isolation before passing a band of 
frequencies from 1.5 to 6.5 MHz to a summing amplifier 81. A bias signal 
generator 82 provides an 8 MHz bias signal to summing amplifier 81. The 
addition of a bias signal to the signal being recorded is conventional and 
improves the ability of the VCR to record lower level signals. 
The summed output of amplfier 81 is provided to an equalizer 83 and a 
recording amplifier 84 which are part of the original VCR. Signals from 
the recording amplifier 84 are provided to a tape recorder mechanism 85 
including recording and reading heads, a tape drive, and a recording tape 
media 89. Signals from tape mechanism 85 are provided to a playback 
amplifier 86 and an equalizer and buffer 87, which are also original parts 
of the VCR. A switch 88 activates either recording amplifier 84 or 
playback amp 86 depending on whether the VCR is in the record or playback 
mode. 
Direct connection to equalizers 83 and 87 of VCR 55 bypasses conventional 
circuitry (not shown) in VCR 55 which tunes and modulates video signals 
normally recorded by VCR 55. Since VCR modulation is by frequency, the FM 
broadcast signals recorded in this invention would be modulated by 
frequency a second time, which is undesirable. Therefore, that circuitry 
is bypassed. 
Turning now to FIG. 8, a method according to the present invention begins 
in step 90 where the modified VCR and circuitry of the present invention 
are installed in a vehicle. The vehicle is driven through a reception area 
containing broadcast signals in step 91. In step 92, the received RF 
broadcast signals are converted to the frequency range of the modified 
VCR. The converted signals are recorded in step 93. 
In order to perform testing and evaluation of radio receiver equipment, the 
recorded media is returned to a fixed location such as a lab in step 94. 
The recorded signals are reproduced in step 95. The reproduced signals are 
reconverted to the original broadcast band in step 96. Finally, the 
reconverted signals are coupled to the input of a receiver or other 
equipment to be tested in step 97. 
Thus, the present invention has provided a method and apparatus for 
receiving and recording RF broadcast signals in a modulated form such that 
recorded data can be played back in a fixed location to analyze data, to 
compare performance of radios under identical (i.e., recorded) test 
conditions, and to aid in development of radio and radio circuits. The 
necessity to conduct frequent field trips at remote locations is 
minimized, thus providing better efficiency in developing diversity radio 
systems and components. 
While preferred embodiments of the invention have been shown and described 
herein, it will be understood that such embodiments are provided by way of 
example only. Numerous variations, changes, and substitutions will occur 
to those skilled in the art without departing from the spirit of the 
invention. Accordingly, it is intended that the appended claims cover all 
such variations as fall within the spirit and scope of the invention.