Superheterodyne Radio Receivers

Transcription

1 EE354 Superheterodyne Handout 1 Superheterodyne Radio Receivers Thus ar in the course, we have investigated two types o receivers or AM signals (shown below): coherent and incoherent. Because broadcast transmissions never occur in isolation (i.e., lots o people want to transmit simultaneously), the only way our receivers can recover inormation rom our desired station is via the use o a tunable Bandpass Filter (BPF) beore the detector stage. The BPF must have a bandwidth narrow enough to select our desired station and reject all others. AM Receiver Architectures s t sˆm t s t sˆm t sc t Commercial AM Radio in the US Frequency Range: khz Station Bandwidth: 10 khz Maximum Audio Frequency: 5 khz Carrier Spacing: 20 khz Consider the ollowing problem. The table above lists the speciications or commercial AM radio in the US. To tune into any given station, the receiver has to have a bandpass ilter with a bandwidth equal to the bandwidth o the transmitted signal. Let s look at the bandwidth o the ilter as a percentage o the carrier requency. In the worst case: BW 10 khz % BW 5.9% 1700kHz c We can also deine something called the Quality Factor o the ilter, also known as the Q o the ilter: c 1700 khz Q 170 BW 10 khz Note that %BW and Q are basically inverses o each other.

2 EE354 Superheterodyne Handout 2 What s the problem? The problem is that Q 10 is extremely hard to achieve. The problem will be much worse as we get to higher center requencies 100 MHz; 1-2 GHz; GHz). The solution: We need a way to shit our signal to a lower requency, where it is much easier to implement a BPF that will reject all other stations. That process is called Heterodyning. Heterodyning In 1918, Edwin Armstrong invented the idea o a Heterodyne receiver. Heterodyning is the translation o a signal rom a higher Radio Frequency () carrier signal to a lower Intermediate Frequency (). Three Important Frequencies in Heterodyne RX Radio Frequency. The center requency the signal is broadcast on. Intermediate Frequency. Fixed requency inside the RX. The signal is downconverted to this requency. Local Oscillator. Tunable requency inside the RX used to translate the signal to the requency. Structure o the Superheterodyne Receiver s t s t ˆm t Section Section Baseband The superheterodynce receiver works by making use o the requency translation properties o the Fourier Transorm: High Side Injection: Low Side Injection:

3 EE354 Superheterodyne Handout 3 Mathematically we can show what happens with a simple AM signal: cos 2 s t A m t ct Using Low-Side Injection, ater mixing, we have: 1 cos 2 cos 2 A 1 cos 2 cos2 s t s t s t s t A m t t t c s t 2 m t c t c t Note: c c will be rejected by the BPF Filter will be our Frequency But. We presumed a -, what happens or a +? I C Note That: o Heterodyning doesn t select one requency, it selects two. o The two requencies are mirrored about the, hence the term Image Frequency. o The primary purpose o the ilter is to reject any signal that might be present on the image requency. See the igure on the next page or a graphical illustration o the image requency problem with superheterodyne receivers.

4 EE354 Superheterodyne Handout 4 Illustration o the Image Frequency Problem with Low/High Side Injection

5 EE354 Superheterodyne Handout 5 Supterheterodyne Example Broadcast AM, 540 khz C For High-Side Injection, determine the Frequency: 540 khz 455kHz 955kHz Now determine the Image Frequency (easiest way to keep the bookkeeping straight is to draw a picture): I 955kHz 455 khz 1450kHz C Note: Always mirrored about I 1450 khz is an In-Band Station! Now, take the same radio and tune to 1450 khz For High-Side Injection, determine the Frequency: 1450 khz 455kHz 1905 khz Now determine the Image: I 1905 khz 455kHz 2360 khz C Note: Always mirrored about I Keep in mind: 2360 khz is outside the AM band, however, some other service has been assigned that requency by the FCC.