The heart of the FCD Pro is the E4000 low power multi-band tuner IC. This IC was designed by a fabless IC design company called Elonics. News releases indicate that the E4000 is manufactured in a modern 90nm mixed-signal CMOS process offered by IBM. Appendix A of An Introduction to the Funcube Dongle provides a basic information sheet for the E4000. Information sheets for the E4000 and other products can also be accessed at the Elonics website. In order to obtain additional technical information, it is necessary to sign a non-disclosure agreement (NDA) with the company. The discussion in this post is based solely on the content of the information sheet.
The E4000 has a zero-IF architecture. This means that it translates a portion of a received RF spectrum directly to baseband frequencies without using an intermediate frequency. Very simple direct conversion receivers that use a single mixer do not reject image frequencies. A well-known phasing technique solves this problem. The basic idea is to use two mixers fed by oscillator signals that are 90 degrees out phase. The resulting baseband signals are referred to as the I (in-phase) and Q (quadrature) signals. Adding and subtracting the I and Q baseband signals allows separation of the upper and lower sidebands and the rejection of unwanted image frequencies. The extent of the image rejection depends on how well the amplitudes and phases of the I and Q paths track.
The E4000 uses this phasing technique. It generates the required oscillator signals on-chip using a PLL-based frequency synthesizer. The information sheet states that the E4000 works over the entire frequency range between 64 MHz and 1700 MHz. However, according to the technical FAQ page of the FCD Pro, ‘there is a gap in coverage between about 1,100MHz and 1,270MHz where the design of the local oscillator VCO, PLL and divider chain in the tuner chip do not provide seamless coverage.’ The same source reports that the FCD Pro has been found to operate at frequencies as low as 51.5 MHz, and at frequencies that exceed 2 GHz.
I believe that the E4000 is very well suited for low-cost, low-power, medium performance, high-volume, TV tuner applications. It is low cost because it uses silicon technology, and low power because it uses a high performance mixed-signal CMOS technology that uses a low supply voltage of 1.5 V. The ability to perform digital and analog operations on the same chip seems to have been exploited effectively. I like the ability to control the gain of individual stages using a simple data bus. It would be interesting to know more about the topology and the electrical performance of the RF mixers, and the implementation and phase noise characteristics of the PLL-based frequency synthesizer. The mixers may be prone to overload, since a 1.5V supply does not allow much headroom.
What is not to like about the E4000? Not much … but there is always something. The E4000 includes a broadband low-noise amplifier (LNA) whose noise figure is <4dB. This is quite good, considering that it is achieved using silicon CMOS, and is probably adequate for consumer applications. However, it is a long way from being state-of-the-art. I will discuss the implications in the next post.